1. Introduction
Construction and Demolition Waste (CDW) includes all inert materials resulting from the execution or demolition of building and civil engineering works [
1]. The nature of this type of waste differs according to its source and collection process [
2]. Source separation is essential for the effective initiation of a recycling and re-use process for these wastes [
3], which in turn enables them to be reintroduced as raw materials into the manufacturing process using circular economy criteria [
4].
Sand is the second most consumed raw material in the world and its scarcity in some countries is expected to have an impact on the development of the construction sector [
5,
6]. For this reason, it is necessary to reconcile the industrial growth of nations with the preservation of the environment [
7], in such a way that an effective recycling route for CDWs is their crushing, grinding and separation to be used as aggregates [
8]. At present, the most direct application of recycled aggregates is the execution of roads [
9], prefabricated for building and civil works [
10], and as a partial replacement for natural aggregate in the production of mortars and concrete [
11]. However, several authors have investigated the properties of these raw materials and have made it possible to extend the field of application of these aggregates for the production of new construction materials [
12,
13].
In this work, two types of recycled aggregates were used for the manufacture of mortars compared to natural aggregates. On the one hand, recycled concrete aggregates were used, so called because they have more than 90% crushed concrete and natural stone without mortar [
14]. Additionally, on the other hand, recycled ceramic aggregates with a percentage of ceramic material not less than 70% are used [
15].
In this way, the most relevant properties of this type of aggregates can be exposed. With regard to aggregate recycling of concrete, some researchers have found that for substitution rates higher than 25% of aggregate in the manufacture of mortars there is a decrease in the mechanical and physical properties of mortars [
16]. This decrease in mortars made from 100% recycled concrete is translated into higher water absorption [
17], higher sulfur content [
18], lower density and compressive strength [
19], less workability of the mixture requiring the use of plasticizers [
20] and more retraction during setting [
21]. Recycled aggregates from ceramic waste, on the other hand, have lower mechanical resistance than recycled concrete aggregates [
22], which has a negative impact on the performance of masonry mortars made from this type of sand. Among the most important characteristics of mortars made with ceramic recycled aggregate are their excessive porosity and demand for kneading water [
23], their high shrinkage values and their high fine content generated during their manufacture [
24], their lightness [
25] and its reduced durability in salt crystallization cycles [
26].
On the other hand, and with the aim of improving the technical performance of mortars made from recycled aggregate, the incorporation of reinforcing fibers in this type of conglomerate materials has been studied during recent decades by several researchers [
27]. In a first classification, reinforcing fibers can be differentiated into synthetic fibers, including, among others, glass fibers, basalt fibers or carbon fibers, and natural fibers such as coconut fibers, hemp fibers or wood fibers. Two synthetic fibers were used in this work: polypropylene and polyolefin. Polypropylene fiber has been used by some researchers to reduce shrinkage by drying, to reduce the possible formation of cracks and to improve mechanical behavior by reducing the fragility of the material [
28,
29]. Among the most recent studies carried out on mortars made from recycled aggregate, the use of this type of fiber to maintain the dimensional stability of mortars during setting is noteworthy [
30], and also those that use this type of reinforcement to reduce cracking in lightweight prefabricated panels that are to be subjected to flexotraction stresses [
31]. On the other hand, polyolefin fibers have traditionally been used for the production of concrete reinforced with fibers, obtaining good results in reducing cracking and increasing the ductility of the material [
32,
33]. Among its most recent applications in the production of recycled mortars, the one made by J.C. Slebi-Acevedo et al. stands out [
34], where, using polyolefin fibers, they have managed to improve mechanical resistance and reduce cracking of mortars made from asphalt residues. These are therefore two reinforcing materials that have proven their applicability in different construction materials and whose study can help to improve the mechanical performance of mortars made from recycled aggregate. Finally, it is worth mentioning the recent research carried out by Cascardi, A. et al. [
35], which highlighted the importance of using connectors to improve the bearing capacity of multi-sheet masonry walls, using different types of materials and connector geometries, proposing an empirical model of great relevance for the construction sector.
In the light of the previous studies described, the aim of this research was to analyze the physical and mechanical properties of mortars made from recycled aggregate and reinforced with synthetic fibers. To this end, different series of mortars were developed with recycled ceramic and concrete aggregates, to evaluate their performance and determine the effect that the incorporation of polypropylene or polyolefin fibers into hardened mortar samples causes. All these results were compared with samples made from natural aggregate, with the aim of establishing a reference series that allows a statistical discussion and conclusions to be drawn based on the properties achieved by traditional mortars.
3. Results and Discussion
3.1. Mechanical Resistance to Flexural and Compression
Figure 3 shows the results of the flexural and compression strength tests carried out on test pieces of 4 × 4 × 16 cm
3 of each of the mixes. The test pieces were tested at the age of 28 days after being cured in a wet chamber under conditions of 95% relative humidity and ambient temperature of 23 °C.
As can be seen in
Figure 3, the bending resistance of traditional mortars made from natural aggregate is higher than that achieved by mortars made from recycled aggregate. However, it can be seen that mortars made from recycled aggregate of concrete have higher bending resistance values than mortars made from ceramic recycled aggregate. There are also differences between the type of synthetic reinforcement fiber added.
Table 9 shows the ANOVA analysis for this property.
As can be seen in
Table 9, the two factors included in the study have a
p-value lower than the level of significance (α = 0.05) and are therefore considered statistically significant.
Below, in
Table 10, the results for the multi-range test for this property are presented.
Thus, in
Table 10, after the analysis of the multirange test, it can be observed that there are significant differences at all levels for each of the factors analyzed. In such a way that the mortars reinforced with fibers present better resistance to bending, the fiber of polypropylene is the better option for reinforcing rather than the fiber of polyolefin. In addition, it can be concluded by saying the RA-Con-1:3-FPP mixes were those that presented bending resistance closer to those achieved by traditional mortars.
Figure 4 shows the results obtained after the compression strength test for the various mix.
It can be seen in
Figure 4 as the mortars made with natural aggregate were the ones that showed the greatest resistance in this trial. In addition, it can be seen how the mortars made with recycled aggregate of concrete that possesses a greater density than the recycled ceramic aggregate, presented superior mechanical resistances. For this property, the addition of synthetic fibers has meant an increase in strength.
Table 11 presents the ANOVA analysis for this property, where it can be seen that all the factors included in the study are significant by presenting a
p-value lower than α = 0.05.
On the other hand,
Table 12 shows the test of multiple ranges.
As can be observed in the test of multiple ranges indicated in
Table 12, the levels “aggregate of concrete” and the type of fiber “Polypropylene” have statistically higher values and therefore the mixes incorporating these levels have a higher compressive strength.
It has been shown that the addition of synthetic fibers in the mold matrix makes it possible to stop cracking in the face of bending forces and improves the internal cohesion of the conglomerate material [
62]. In addition, the compression resistances obtained are indicative of the quality of mortars made from recycled aggregate and reinforced with fibers, since they provide information on the capacity of the materials tested when it comes to bearing loads without breaking up [
63]. In this way, it has been possible to observe how the addition of synthetic fibers in the percentages indicated in the research improves the mechanical strength of the mortars; similar results have also been obtained by other researchers who used natural fibers during the mixing process to improve the mechanical properties in this type of material [
64].
To better observe this cohesion between the recycled aggregate and the conglomerate, and between fiber and mortar matrix,
Figure 5 shows microscopy images of the RA-Con and RA-Con-1:3-FPP-type mixes.
As can be seen in
Figure 5a, there is a correct setting and hardening of the cementitious material that is reflected in the formation of Ettringite crystals in the mortar matrix made from recycled aggregate. On the other hand,
Figure 5b also corresponding to test piece RA-Con-1:3 shows the good cohesion in the interface between the recycled aggregate of concrete and the conglomerating material, this has an impact on better compressive strength and less segregation of aggregates [
65].
Figure 5c,d show the interface between the polypropylene fiber and the mortar matrix in the RA-Con-1:3-FPP test pieces, which achieved the best bending results. It can be seen that there is a homogeneous distribution of the fibers inside the mortar and that they are well adhered.
3.2. Physical Properties and Other Tests
Table 13 shows the results corresponding to the other physical properties analyzed: surface hardness, bulk density, adhesion and water absorption by capillarity.
Table 13 shows that the mortars made with natural aggregate were the best results for the physical properties analyzed. With regard to the analysis of recycled mortars, firstly, the surface hardness Shore D was higher in the test pieces incorporating recycled aggregate of concrete, where it is also noted that the incorporation of fibers does not represent a significant improvement in this property. On the other hand, the bulk density was also higher in the test pieces incorporating the recycled aggregate of concrete and reinforced with fibers; this is due to the higher density of recycled concrete aggregates with respect to recycled ceramic aggregates as can be seen in
Table 2.
In terms of adhesion, it has been shown that the incorporation of fibers does not improve the results for this property. The values reached for the adhesion tests of the mortar on a ceramic base reflect that the incorporation of recycled concrete aggregate in recycled mortars does not imply a variation for this property with respect to the incorporation of ceramic recycled aggregate, because in any case, it can be seen that the mortars made with ceramic recycled aggregate presented slightly higher values. Finally, the absorption of water by capillarity is much higher in mortars made with recycled aggregate compared to those made with traditional mortars, with the mortar made with ceramic recycled aggregate having the highest absorption coefficients.
In addition, a statistical analysis was carried out to verify whether or not there are statistically significant differences in all the physical properties studied.
Table 14 shows the ANOVA test values for a significance level of α = 0.05 in all properties. Additionally, on the other hand,
Table 15 shows the results obtained after performing the test of multiple ranges for the different factors and levels.
As can be seen from the analysis in
Table 14, for the properties of adhesion, surface hardness and water absorption by capillarity, the only significant factor is the type of aggregate. This is not the case with density, where statistical significance for the fiber factor can be seen. In the same way, looking at
Table 15, it can be appreciated how the mortars made with recycled aggregate of concrete have presented better results before the physical properties studied, the type of fiber added does not substantially improve the fiber content except in the case of bulk density.
In addition,
Figure 6 shows the results obtained for the dimensional variation expressed as a percentage of shortening, which is caused by the retraction during the setting in the different mixes studied.
As can be seen from the analysis in
Figure 6, traditional mortars made from natural aggregate have greater dimensional stability and less shrinkage than mortars made from recycled aggregate. Within these recycled mortars, those made with ceramic recycled aggregate had higher shrinkage values than their counterparts made with recycled aggregate of concrete. Thus, it can be verified that, in accordance with other studies, the incorporation of this type of CDW in the manufacture of mortars increases the shrinkage in mortars [
66].
However, these retraction values are reduced with the incorporation of fibers in the mortar matrix, with better results presented for the addition of polypropylene fiber rather than polyolefin fiber. In fact, the RA-Con-1:3-FPP dosage is the one that best approximates its behavior to the mortar made with natural aggregate.
3.3. Statistical Discussion. Confidence Intervals for Mean Difference
Finally, confidence intervals were calculated for the difference in mean to 95% confidence in order to determine the technical competitiveness of the studies carried out for this research. These ranges were constructed to assess the differences between traditional mortars (NA-1:3) with respect to mortars made from recycled aggregate and polypropylene fiber (RA-Con-1:3-FPP), and to analyze the differences in means between the latter and the mortars made with recycled aggregate and without fibers (RA-Con-1:3). These dosages were chosen based on the analysis of the variance performed, since they were the best results for the different trials.
In addition, for the performance of this statistical analysis it was necessary to increase the sample size up to 30 specimens of each type of mortar analyzed to obtain reliable results.
The intervals obtained in this analysis are shown in
Table 16, arranged according to each property evaluated.
To analyze the confidence intervals calculated in
Table 16, it should be noted that the difference in means has always been calculated as the minus of
at 95% confidence. From the comparison between the mixes RA-Con-1:3 and RA-Con-1:3-FPP, a significant improvement in the performance of the mortars incorporating polypropylene fibers in terms of bending and compression resistance is observed, with the mixes able to achieve improvements of around 0.70 and 1.2 MPa, respectively. In the rest of the tested properties, a behavior similar to recycled mortars that do not incorporate fibers is observed.
On the other hand, the difference in calculated means between mortars made from natural aggregate and recycled mortars that incorporate polypropylene fibers shows a better performance in traditional mortars in all the tested properties. However, in the resistance to bending and compression, the difference in performance is reduced between both types of mortar, justifying the incorporation of fibers in recycled mortars to increase their competitiveness as masonry mortars.
4. Conclusions
In this work, the mechanical properties of cement mortars reinforced with synthetic fibers and made with recycled aggregate were analyzed. The technical feasibility of these materials for use in the construction sector as masonry mortars was verified. In this way, this study has contributed with the provision of technical information of interest to construction professionals who wish to use more sustainable and environmentally friendly materials, thus contributing to achieving the objective of promoting cleaner production through the efficient use of natural resources, as set out in the European 2030 Agenda.
As for the mechanical properties, the tests carried out show that the recycled aggregate of concrete allows obtaining mortars with better results in terms of resistance to bending and compression, in comparison with the mortars made with ceramic recycled aggregate. In addition, polypropylene fiber is presented as a better synthetic reinforcement material than polyolefin fiber to improve the mechanical properties of recycled mortars. However, in all cases analyzed, traditional mortars have performed better than recycled mortars. On the other hand, the correct setting of the mortars made with recycled aggregate was demonstrated, as well as its good internal cohesion and good adhesion between the fibers and the mortar matrix thanks to the analysis by electron microscopy of low flow.
With regard to the other physical properties analyzed—surface hardness, adhesion, bulk density, capillary water absorption and shrinkage—traditional mortars with natural aggregate are still the ones with the best results. As for hardness, the incorporation of fibers does not imply a modification of this property and better results were presented for mortars with aggregate recycled concrete. Regarding the adhesion and absorption of water by capillarity, the mortars made with ceramic recycled aggregate presented higher values for this property and were also lighter than the mortars made with recycled aggregate of concrete. Finally, the positive effect of the incorporation of synthetic fibers in the mortar matrix in reducing shrinkage during setting was verified. In this sense, mortars with recycled concrete aggregate have a less retraction than mortars with ceramic recycled aggregate, and this shrinkage for both types of recycled mortars is reduced more by the incorporation of polypropylene fibers than by the incorporation of polyolefin fibers.