**1. Introduction**

The end-of-life tyre (ELT) is a type of waste that has become an environmental and social problem. In effect, the accumulation of ELT produces concentration of rats, larvae, mice, insects, and it increases the risk of fires difficult to extinguish [1].

Moreover, globally, 1000 million tonnes of ELTs are generated annually, with more than 50% destined to landfills or left as untreated garbage [2]. In Chile, for instance, more than 145,000 tonnes of ELT were generated in 2019 and only 17% was recycled [3,4]. In this regard, efforts have been made to improve the management of this waste through Law N◦20920, which establishes the framework for waste management, extends producer responsibility, and the promotion of recycling in Chile [5]. Although, in this law, tyres are defined as priority products, there is no special provision for the management of ELTs. Therefore, the problems associated with ELT disposal remain.

On the other hand, concrete is a relatively inexpensive material, with the ability to develop high strengths and different shapes, which makes it suitable for multiple

**Citation:** García, E.; Villa, B.; Pradena, M.; Urbano, B.; Campos-Requena, V.H.; Medina, C.; Flores, P. Experimental Evaluation of Cement Mortars with End-of-Life Tyres Exposed to Different Surface Treatments. *Crystals* **2021**, *11*, 552. https://doi.org/10.3390/cryst11050552

Academic Editors: Cesare Signorini, Antonella Sola, Sumit Chakraborty and Valentina Volpini

Received: 1 April 2021 Accepted: 3 May 2021 Published: 15 May 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

applications. In fact, concrete is the most widely used material in the construction industry worldwide, producing approximately 25 billion tonnes annually [6].

However, the concrete manufacturing process requires large amounts of energy, raw materials, and has a large impact on the environment. Therefore, alternatives have been developed to reduce these impacts through the use of non-conventional materials, such as recycled waste. Among the alternatives developed, the behaviour of concrete with added rubber has been studied by different authors [7–12]. These studies report that the inclusion of rubber can improve properties of the concrete, such as energy absorption capacity, ductility, thermal insulation, resistance to lead cycles, post-cracking behaviour, and durability [7–10,13]. Considering these possibilities of improvement and the massive use of concrete, the study of the incorporation of ELT in the concrete material is very attractive as a solution for reusing this waste.

However, although the potential benefits of adding ELT rubber into the concrete, there are challenges related to the weak rubber-cementitious matrix interaction, which results in basic mechanical concrete properties negatively affected [9,11,12]. Furthermore, the dependence of the concrete properties on the individual components, which vary from region-to-region [14], makes local studies on the impact of incorporating ELT in concrete mixes necessary. For instance, evaluations of fresh concrete mixes indicate an increase in workability [15,16], while others report reductions in this property [17–19], and even cases of no appreciable change [20,21]. Moreover, studies show that it has not been possible to find a relationship between replacement percentage and grain size of ELT with the mix workability [22]. However, other studies report such relationships. For instance, the results of Su et al. [23] indicate that concretes with larger rubber show better workability than those with smaller rubber particles. Additionally, the case of concrete with continuous rubber granulometry offers better workability and strength to water permeability compared to concretes with single rubber size.

Additionally, different investigations conclude that rubber incorporation can have a negative impact on the basic mechanical concrete properties, i.e., the flexural and compressive strength [7–9,13,24–27]. Indeed, Liu et al. [28] studied the performance of rubber-based concretes at replacement percentages of 5%, 10%, and 15% with respect to fine aggregate volume. The results indicate that, compared to standard concrete specimens, the rubberincorporated specimens decrease the compressive and flexural strength. Moreover, the reported decrease in compressive strength is twice the reduction in flexural strength. However, in general, the experimental results show a significant improvement in the cracking behaviour of the material, together with a higher resistance to load cycles and higher toughness.

Yu and Zhu [24] report that rubber content and size can affect the porosity structures of cement mortars. Actually, they state that the reasons behind the reduced strength are the combined changes in rubber content and porosity structures.

In order to minimise the reduction of the mechanical properties of cementitious materials, the application of rubber treatments before the addition to the mix has been evaluated [25–27]. For instance, Mohammad et al. [26] developed a treatment consisting of soaking ELT rubber in water for 24 h, before adding it to the rest of the concrete components. This treatment reduces the amount of air trapped on the surface of the material, which, after the hydration time, produces a decrease in the amount of air bubbles around the material. The result is a relatively minor concrete strength reduction, being more favourable the results of compressive strength than flexural strength. Furthermore, samples with replacement less than 20% showed improvements of the fatigue strength under load cycles [26].

Another treatment developed consists of an oxidation process using a solution of potassium permanganate (KMnO4) and sulphonated with sodium bisulphite (NaHSO3), at different concentrations and contact times. The results showed that the treatment modified the rubber surface, decreasing the contact angle of the rubber with water and significantly improving the interaction between the cementitious matrix and the rubber particles. Therefore, properties, such as compressive strength and impact resistance, were

also positively affected. The strength improvements of rubber modified concrete as a function of the rubber percentage was evident and accentuated at around 4% rubber content, with an improvement close to 10 MPa [27].

However, the properties of concrete mixes with rubber are locally dependent due to differences in cement manufacture and components, aggregate characteristics, and ELT properties.

In fact, geo-dependency is one of the characteristics that help to explain the massive use of concrete. Geo-dependency as well is crucial when dealing with waste materials and concrete, especially because the alternative solutions must be practical and feasible to implement in order to effectively reuse the waste [29].

The work presented in this article is part of a wider investigation on the effects of ELT rubber in the concrete material. In particular, and considering factors as geo-dependency and the complexity of rubber treatments, this stage focuses on evaluating traditional properties of mortars with ELT rubber using Chilean cements. More specifically, the objective of the present study is to characterise the properties of cement mortars with the addition of ELT rubber under three surface treatments. The study considers different variables to define the substitution percentages and the appropriate rubber granulometries according to the performance of mortar properties in the fresh and hardened state. In this way, it is expected to define mortar mixes with the best mechanical performance considering technical, practical, and economic aspects.

#### **2. Materials and Methods**

*2.1. Materials*

2.1.1. Cement

Two commercial brands of cement available in the local market were used in this research: Bío Bío Especial (C1) and Polpaico Especial (C2). This choice was based in the necessity of proposing practical and sustainable alternatives for the effective use of ELT rubber.

According to the standard NCh 148, based on ASTM C150/C150M-20, C1 and C2 cements were classified as standard grade Portland pozzolanic cement [30]. The properties of the cements used in this study are presented in Table 1.


**Table 1.** Cement characterization.

One way to chemically characterize the cement is in terms of its oxide's composition, which is directly related to the final properties that a mortar or concrete can develop. Tapia [31] performed a chemical analysis in terms of the oxide components of the cements, including the two cements used in this research (Table 2). Calcium oxide (CaO) and silicon oxide (SiO2) are the most important for this research, since, from them, dicalcium silicate (C2S) and tricalcium silicate (C3S) are formed, which are the main components of the clinker and they are responsible for the strength of the hydrated cement paste.


**Table 2.** Oxide components of cements, in percentage [31].

Furthermore, Tapia [31] made a SEM–EDX analysis where the presence of fly ash in the cement C1 was evidenced, the fly ash being an agent known for the increment of the concrete strength on time.

#### 2.1.2. Sand

Sand consists of a stone material composed of hard particles with a stable shape and size that pass through the 4.75 mm aperture sieve and it is retained on the 0.075 mm sieve [32]. In the present study, Bío Bío sand, available at the Concrete Laboratory, Universidad de Concepción, was used. Bío Bío sand is the typical fine aggregate for making concrete in the local market. The sand properties according to the standards NCh 1239:2009 [33], based on ISO 7033:1987, and NCh 165:2009 [34] based on ISO/DIS 20290-1, are presented in Table 3.

**Table 3.** Sand characterization.


#### 2.1.3. Water

The water used was from the public water supply that complied with NCh 409/1, and was not contaminated prior to use [35].

#### 2.1.4. Rubber

The rubber was provided by the company Polambiente, which recycles ELT from truck and car tyres. Once received, it was sieved in order to obtain three different sizes, which are presented in Table 4.

#### **Table 4.** Rubber particle sizes.


Each of these sizes was characterised using the Chilean regulations associated with fine aggregate [32,33], which is shown in Table 5.

