*2.2. ELT Modification*

In the search for improving the adhesion and interaction between the cementitious matrix and the ELT rubber aggregate, three methods were applied to the surface of the ELT grains before the incorporation in the cement mortar mix.

#### 2.2.1. Treatment 1: Hydration (R1)

Treatment 1 is the simplest of the three treatments applied. The method is based on the work by Mohammadi et al. [26], and consists of soaking the rubber in water for 24 h before incorporating it into the mortar mix. This treatment reduces the amount of air trapped on the surface of the material after the hydration time, which improves the interaction with the cement paste.

#### 2.2.2. Treatment 2: Oxidation–Sulphonation (R2)

Treatment 2 is based on the work by He et al. [27]. It consists of an immersion of the rubber in a 15% sodium hydroxide solution stirring at 150 rpm for 45 h at room temperature. After washing and filtering, the sample is put in contact with a 7% KMnO4 solution. This solution is adjusted to pH 2 and stirred at 150 rpm for 2 h at 70 ◦C. The product is then transferred to a saturated NaHSO3 solution, which is reacted at 150 rpm for another 2 h at 70 ◦C. After washing and filtering, the sample is dried for 12 h at 50 ◦C. By this means, polar groups, such as carbonyl, hydroxyl, and sulfonic groups, are introduced on the surface of the rubber, which increases the hydrophilicity.

#### 2.2.3. Treatment 3: Contact with Hydrogen Peroxide (R3)

This method is based on the study by Shatanawi et al. [25] originally developed to incorporate rubber into asphalt mixes. The treatment consists of contacting the rubber with a hydrogen peroxide solution at a concentration of 50% at a temperature of 60 ◦C, for a time of 60 min, where a Fenton reaction is subsequently provoked, using a concentration of Fe2+ 12 mM and H2O2 600 mM at pH 3–4. These reactions produce homolytic cleavage of single C–C bonds to produce terminal hydroxyls.

#### *2.3. Experimental Program*

Due to the large number of combinations, and in order to reduce the uncertainty of the results, the evaluation was divided into three stages. The variables considered in each phase were the type of cement, the size of the rubber grains, the surface treatment applied to the rubber, and the test age to which the samples were subjected. Moreover, the percentage of rubber replacement varies from a range of 0% (control samples) to 12.5%, with respect to the weight of the fine aggregate.

The first stage aimed to make a fast characterization of the samples in order to take useful decisions for the next phases. For that, samples with the addition of untreated rubber, at early test ages of 7 and 14 days, with two cements, were included in the experimental analysis. Furthermore, two replacement percentages were considered, a minimum of 2.5% and a maximum of 12.5%, with respect to the weight of the fine aggregate and with three rubber grain sizes (T1, T2, and T3). Figure 1 presents the details of stage 1.

**Figure 1.** Experimental plan description of stage 1.

Based on the results obtained in the first stage, the second stage is developed (Figure 2). This phase contemplates substitution percentages with rubber of 5% and 7.5% with respect to the weight of the fine aggregate in conditions without treatment and with treatment R1, and test ages up to 28 days. This, in order to have more information on the properties of the samples studied, maintaining the two cements and the three sizes of rubber as variables.

**Figure 2.** Experimental plan description of stage 2.

Due to the technical complexity of applying the R2 and R3 treatments, stage 3 is based on the previous results in order to optimize the number of samples evaluated. Therefore, in this stage, the best results obtained in the previous stages are considered in the analysis. Figure 3 presents the details of stage 3.

**Figure 3.** Experimental plan description of stage 3.

In the three stages, the tests were conducted on specimens with dimensions 40 × 40 × 160 mm according to the Réunion Internationale des Laboratoires et Experts des Maté-riaux, systèmes de construction et ouvrages (RILEM) [36]. The workability of each mixture was evaluated according to the NCh 2257/3 standard [37], based on ISO 1920-2: 2016, using the reduced cone method. Additionally, the flexural and compressive strengths

were evaluated according to the standard NCh 158 [38], whose international equivalent corresponds to the UNE-EN 196-1: 2018 standard. In order to determine the density, the weight and size measurements of the samples were performed at the hardened state in accordance to NCh 158 [38]. In addition to the macro-characterisation of the samples, i.e., the fresh and hardened mechanical behaviour of cement mortars, a micro-level analysis of the rubber grains under the treatments was performed. The micro-characterisation consists of the measurement of the contact angle between the rubber samples and the water, in addition to the analysis of the SEM (Scanning Electron Microscopy) images. The aim of this approach is to have a better understanding of the macro behaviour of the cement mortar samples.

#### *2.4. Cement Mortar Mix Dosage*

For the mix dosage, the Mortar Manual developed by the Chilean Institute of Cement and Concrete was applied. This manual establishes generalities, properties and dosage methods for cement mortars [39]. The procedure based on compliance with workability and compressive strength, modified due to the partial replacement of the aggregate by ELT rubber, was applied.

The reference compressive strength used in this study corresponds to 300 kg/cm2, which, according to the applied design method [39], implies a design strength of 350 [kg/cm2], with a medium workability, which means a drop of between 3 and 8 cm measured by the reduced cone method. The mix dosage is presented in Table 6.

**Table 6.** Mix dosage to produce 1 m3 of mortar.


#### **3. Results**

*3.1. Stage 1*

3.1.1. Workability

The results of the reduced cone test for the samples with untreated rubber addition are presented in Figure 4. With respect to the control mortar, without rubber addition, there is an important variation in the workability of the mix with high dependence on the percentage of replacement. With larger amounts of rubber, although all the results are within the design range [38], the samples with smaller rubber size have a much lower workability compared to the others. This is explained due to the higher specific surface area of the smaller rubber particles, causing a greater friction with the rest of the mixture, resulting in a lower cone slump.

**Figure 4.** Mortar cone slump with addition of untreated rubber 2.5% and 12.5% replacement: (**a**) cement C1; (**b**) cement C2.

#### 3.1.2. Density

The results of the densities for the samples with the addition of untreated rubber are presented in Figure 5. The error bars correspond to the standard deviation for the two cements used; the measured densities are similar, with minimal differences of around 2% in the case of the control mortar, being generally higher in samples with cement C1. Due to the lower density of the ELT rubber with respect to the aggregate, an inverse relationship is observed between the amount of rubber added and the mortar density. Regarding the size of the rubber aggregate, it is not possible to establish a relationship between this and the density since the results do not show a trend in this respect.

**Figure 5.** Density of mortar with addition of untreated rubber 2.5% and 12.5% replacement: (**a**) cement C1; (**b**) cement C2.

#### 3.1.3. Flexural Strength

The results of the flexural strength for the samples with the addition of untreated rubber are presented in Figure 6. The control mortar has practically equal strengths with both cements at 7 days. This changes at 14 days, where the samples with cement C1 have higher strengths. It is observed that, as the percentage of ELT rubber increases, the strength of the samples decreases.

At a test age of 14 days, when the cement used is C1, the average strength reduction (considering the three rubber sizes) for replacement percentages of 2.5% and 12.5% is 20% and 39%, respectively, with respect to the control mortar. In the case of mortars made with C2 cement, the reduction in strength for both replacement percentages reaches 15% and 38%, respectively.

**Figure 6.** Flexural strength of mortar samples with addition of untreated rubber at 2.5% and 12.5% replacement, after 7 and 14 days: (**a**) cement C1; (**b**) cement C2.

#### 3.1.4. Compressive Strength

Figure 7 presents the results of the compressive strength tests for the samples with untreated rubber addition. Similar to the results previously shown, a strength reduction is observed as the amount of rubber added increases. In general, the strength reduction is greater than in the case of flexural strength, which is in agreement with what is found in the literature [23]. At a test age of 14 days, when the cement used is C1, the average strength reduction (considering the three rubber sizes) for replacement percentages of 2.5% and 12.5% is 18% and 45%, respectively, with respect to the control mortar. In the case of mortars made with C2 cement, the reduction in strength for both replacement percentages reaches 19% and 48%, respectively.

**Figure 7.** Compressive strength of mortar samples with untreated rubber addition at 2.5% and 12.5% replacement, after 7 days and 14 days: (**a**) cement C1; (**b**) cement C2.

From the results, is possible to conclude that the 12.5% replacement is not recommended. In the case of cement C2, for example, the reduction of compressive strength is approximately 50% with this replacement.
