• Treatment R2

Figure 24 characterises the morphological aspects of the modified rubber surface under the second treatment, showing a surface with some roughness and pores, but still less than the unmodified rubber.

**Figure 24.** Rubber surface under treatment R2.

• Treatment R3

Figure 25 shows the modified surface for experiment 3, with a rough surface composed of small cavities.

**Figure 25.** Rubber surface under treatment R3.

When comparing the surfaces of the unmodified rubber with respect to the three treatments, it is possible to observe that these last surfaces are slightly smoother and with fewer pores, especially regarding to the first and second treatment. The wettability of the surfaces not only depends on their materiality, but on their surface at the micro level as well. In this way, the more irregular the surface, the more hydrophobic it is due to the interaction surface-water [43]. Thus, under these treatments, the surface irregularity at the micro level decreases the hydrophobicity of the rubber, allowing a better interaction between the rubber and the cementitious matrix. The contact angle measurements present trends in the same direction. Although the contact angle is not greatly reduced, under the first (R1) and second treatment (R2) it decreases more than R3, which shows a slight improvement in the interaction with water with respect to the untreated rubber. This is more clearly observed with the treatment R2, where the contact angle decreases by more than 30◦ and is the one with the best result, in terms of mechanical resistance, together with the treatment R1.

Hence, the analysis at the micro level with the contact angle and the SEM images, provide further explanation to the macro performance observed in the cement mortar samples. In this way, the better behaviour of treatments R1 and R2 is confirmed.

#### **4. Discussion**

The results indicate that the addition of ELT recycled rubber modifies the behaviour of the cement mortar. The changes include a reduction of compressive and flexural strength as the rubber is added to the mix. Although this effect is undesirable, it is possible to minimise its impact with the application of rubber treatments, such as those shown in this work.

Although the percentage of rubber added in the concrete mix is low, the large quantities of concrete required in the construction industry can lead to a high total volume of ELT rubber used. For that is important to fulfil the strength design, which is the case presented in this work, without adding, as other investigations, a higher amount of cement [44] or additives [45,46] that reduce the loss of strength. Actually, in this paper, only a treatment on the surface of the rubber that is practical, easy to apply, effective, and economically feasible is proposed. In effect, the results of this study show that is possible to use this waste material while maintaining the design strength requirements, which is different from other investigations, where adding ELT rubber decreases the strength under the designed one [44,47]. Considering the importance of the concrete strength, the promissory results presented in this article open the way to numerous real applications in opposition to the limitations found in other investigations [48,49].

Therefore, although two of the treatments may present similar results in terms of mortar strength, when comparing various factors, such as duration, difficulty of application, and costs, the R1 treatment is more favourable. Particularly, this last aspect is fundamental to develop cost-effective alternatives due to the fact that, although adding ELT rubber can improve different concrete properties, there is also a related cost involved.

Although there is a trend in favour of the T1 ELT rubber size, its election is complemented by other aspects, such as the energy and costs required for the grains production, the smaller the size, the longer the crushing time, which implies higher costs and energy. Another important aspect is the ease with which the rubber grain to be treated can be processed. In the laboratory, when the R1 treatment was applied, the smallest rubber grains did not submerge in the water, as was the case with most of the larger grains at the end of the contact time. Hence, the smallest grain sizes have difficulties to receive the hydration treatment.

This stage of the research focusses in cement mortars, which are a particular case of the concrete material. This allows considering a large number of cases, including different relevant variables, as rubber size, percentage of substitution, rubber treatments, and types of cements, and their influences in fundamental properties as the compressive and flexural strength. This is related with an effective and realistic use of the waste end-of-life tyres, which is part of the motivation of this article. Actually, the composite mortar-waste can have excellent behaviour in other properties, but if the composite material strength is significantly reduced, the practical possibilities to be massively used can be very limited. Considering the promissory results obtained in the present phase of the research, it is recommended to continue investigating other composite properties as ductility, long-term durability, and behaviour at elevated temperatures, among others.

Finally, if the final purpose is to effectively use waste, it is important to consider the geo-dependency of the concrete material [29]. This is particularly evident in the case of cement C1, which is abundant in the Chilean market at a similar cost of cement C2. However, cement C1 is capable of delivering mortars that, despite the strength reduction, they can satisfy the design requirements. Furthermore, cement C1 includes fly ash, which reduces the amount of clinker whilst is reusing a local industrial waste. In this sense, the results obtained in this investigation can be useful for other regions as well. For instance, where it is possible to replace part of the cement by fly ash (produced or imported), or other cement substitute, producing similar results. If the substitute is a waste as the fly ash, it will be important not only from a technical perspective, but also as a contribution to a sustainable development.

#### **5. Conclusions**

The properties and behaviour of mortars are modified when recycled rubber is used as partial replacement of the aggregate. The tests and measurements carried out show that is possible to add ELT rubber up to 5% with respect to the weight of the aggregate. Of the three treatments analysed, the hydration of the rubber is the best option from a technical, practical, and economic point of view. Regarding to the three rubber sizes evaluated, the T1 (2.36–4.75 mm), which is close to the maximum aggregate size, is recommended. This is due to the favourable results in terms of strength, handling, and less energy involved in crushing the ELT. Additionally, cement C1 offers the best mortar results due to its composition, which includes another waste as the fly ash, one of the main responsible for the strength differences at 28 days. In effect, cement C1 not only gives higher compressive strength than C2, but also allows fulfilling the designed strength with the addition of ELT rubber.

In the fresh state, the workability of the mixture depends on the amount and size of rubber added, as well as the surface treatment applied. In general, with larger grain sizes, the workability tends to remain the same or increase, as the amount of rubber increases. However, it is important to highlight that this workability changes remain in the design range.

As expected, the density of the samples with the addition of ELT recycled rubber decreases as the rubber content increases, due to the lower density of ELT recycled rubber with respect to the aggregate. This behaviour depends as well on the size of the rubber grains, due to the specific surface. Indeed, considering the low compatibility between rubber and water, higher presence of entrapped air in the mix is expected due to a larger and irregular surface area generated when the grain size decreases.

The mechanical strength of the samples studied decreases as the percentage of ELT rubber replacement increases. At early age (7 days), the comparative results between mortars made with both cements and untreated ELT rubber are slightly similar. The results are different at 14 and 28 days, where the behaviour of the mortars with cement C1 are always better. For this reason, the third stage of optimization, only considers the cement C1.

Although the three applied treatments modify the hydrophobic nature of rubber, the best results are obtained with treatment R1 (hydration) and treatment R2 (oxidation– sulphonation). The behaviours at macro level can be explained at the micro level, due to the contact angle measurements and SEM images analysis. These results indicate that the treatments are able to alter the surface roughness and the contact angle between surface and water, improving the interaction between the ELT rubber and the cementitious matrix. This is particularly relevant on treatments R1 and R2, which is coherent with the macro results obtained of the cement mortar samples.

Treatments R1 and R2 provide the best results, being these ones very similar between them. However, treatment R1 is much less difficult to apply in practice, due to the fact that R1 uses elements and substances easily accessible at lower cost. Hence, in addition to the technical performance mainly related with the compressive strength is important to consider, as well, the economical, practical, and environmental aspects of the treatments. In this sense, the R1 treatment clearly has advantages over the R2 one, as the ELT rubber hydration is a very economical and practical procedure, which uses only water, i.e., an eco-friendly alternative.

Finally, when C1 cement in conjunction with the R1 treatment are used, the resulting cement mortars with ELT rubber are capable of exceeding the designed strength without adding, as other investigations, more cement, or using additives to reduce loss of strength. Considering the importance of the concrete strength, the promissory results presented in this article open the way to numerous real applications in opposition to the limitations found in other investigations related with concrete and/or mortar incorporating ELT rubber. Hence, the obtained results open the way to numerous practical applications, and then effective uses of the waste end-of-life tyres.

**Author Contributions:** Conceptualization, M.P., B.U., V.H.C.-R., C.M., and P.F.; methodology, M.P., B.U., V.H.C.-R., C.M., and P.F.; validation, M.P.; formal analysis, M.P., B.U., V.H.C.-R., and P.F.; investigation, E.G., and B.V.; resources, B.U., C.M., and P.F.; writing—original draft preparation, E.G. and B.V.; writing—review and editing, M.P.; supervision, M.P., B.U., and V.H.C.-R.; project administration, M.P.; funding acquisition, M.P., B.U., C.M., and P.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by ANID PIA/Apoyo CCTE AFB170007 and VRID Multidisciplinario 219.091.051-M.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Raw data of this paper will be available from corresponding author, M.P., on a reasonable request.

**Acknowledgments:** The authors acknowledge the contribution of the company "Polambiente" for providing the ELT rubber used in this study.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

