*3.2. Stage 2* 3.2.1. Workability

Figures 8 and 9 show the slump measurements of mortar samples with untreated rubber and under treatment R1 (hydration), respectively. When untreated rubber is added to the mix, the trend is similar with both cements, i.e., the workability tends to be maintained or increase with larger rubber sizes. For the T3 size, the workability decreases as more rubber is added. Again, it should be noted that, in all cases, the workability remains within the design range [39] and the general relationship is that the larger the rubber size, the higher the workability.

**Figure 8.** Mortar cone slump with addition of untreated rubber, at 5% and 7.5% replacement: (**a**) cement C1; (**b**) cement C2.

**Figure 9.** Rubber-added mortar cone slump under treatment R1, at 5% and 7.5% replacement: (**a**) cement C1; (**b**) cement C2.

When rubber is added to the mortar under hydration treatment, workability tends to decrease as the amount of ELT added increases. When the replacement percentage is 5%, considering both cements, workability tends to be maintained for samples with larger rubber grain sizes. For samples with rubber size T3 (0.3–1.18 mm), the workability decreases drastically. In the case of 7.5% replacement, this behaviour is repeated and the reduction of workability is accentuated with rubber size T3, reaching the minimum of the design range of 3 cm [39].

## 3.2.2. Density

The densities for the samples with untreated rubber are presented in Figure 10. The results for the samples with cement C1 show that with rubber incorporated at 5% and 7.5%, the density decreases by 2% and 8%, respectively. Similarly, in samples with cement C2 under the same conditions, the density decreases by 3% and 10%, respectively. This fact shows an inverse relationship between the amount of rubber added and the density of the mortar. Regarding the size of the incorporated rubber, in the case of cement C1 mortars, the densities are higher when the rubber used is larger (T1). In the case of cement C2 mortars, medium size rubber mortars (T2) have slightly higher densities.

**Figure 10.** Density of mortar with addition of untreated rubber at 5% and 7.5% replacement, after 14 days: (**a**) cement C1; (**b**) cement C2.

In the case of mortars with rubber addition under treatment R1, it is observed that the smaller the rubber size produces lower densities (Figure 11). For the samples of cement C1 with percentages of 5% and 7.5% of rubber incorporated, the density reductions are on average 3% and 5%, respectively. For the same percentages of incorporated rubber, but with cement C2 samples, the density reductions are 5% and 7% respectively.

**Figure 11.** Density of rubber-added mortar under treatment R1 at 5% and 7.5% replacement, after 14 days: (**a**) cement C1; (**b**) cement C2.

#### 3.2.3. Flexural Strength

The results of the flexural strength tests for the samples with untreated rubber addition and under treatment R1 are presented in Figures 12 and 13, respectively. When comparing the samples without rubber, at early ages, the strength is similar for both cements. However, as the test age advances, the difference increases, with the samples of cement C1 having higher strength.

**Figure 12.** Flexural strength of mortar samples with addition of untreated rubber at 5% and 7.5% replacement, after 7, 14, and 28 days: (**a**) cement C1; (**b**) cement C2.

**Figure 13.** Flexural strength of rubber-added mortar samples under treatment R1 at 5% and 7.5% replacement, after 7, 14, and 28 days: (**a**) cement C1; (**b**) cement C2.

In the samples with untreated rubber, there is no clear trend about the rubber size giving the best results. However, for samples with cement C1, the T1 size shows similar or slightly higher strengths than the other grain sizes. In the case of cement C2, the T3 and T1 sizes showed the highest strengths.

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 5% and 7.5% is 23% and 30%, 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 29%, respectively. Although the strength reduction of samples with cement C2 is smaller compared to the results of samples with C1, in absolute terms, the strength of these last samples is higher in all cases.

The flexural strength of the mortars with rubber under R1 treatment has a similar behaviour to the one observed in stage 1, i.e., a slight improvement in strength with respect to the samples with untreated rubber. This fact is observed to a greater extent in the samples with cement C1. In this case, at a test age of 14 days, when the cement C1 is used, the average strength reduction (considering the three rubber sizes) for replacement percentages of 5% and 7.5% is 13% and 21%, 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 22%, respectively.

#### 3.2.4. Compressive Strength

The results of the compressive strength tests for the samples with untreated rubber addition and under treatment R1 (hydration) are shown in Figures 14 and 15, respectively. At a test age of 14 days, when the cement used is C1, the average reduction in strength (considering the three rubber sizes) for replacement percentages of 5% and 7.5% is 18% and 31%, 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 34% and 39%, respectively. There is a clear difference between the results of the two cements, being the samples made with cement C1, the ones with the highest strength at 28 days. In all cases, it is observed that the strength of the samples containing untreated rubber is higher when cement C1 is used. In addition, the only case of samples containing ELT and fulfilling the design strength of 350 (kg/cm2) is with 5% replacement ELT and size T1.

The results of rubber samples made with cement C1, and under treatment R1, indicate a slight increase in strength compared to the untreated rubber samples. This does not occur in the samples made with cement C2. In this case, 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 5% and 7.5% is 9% and 23%, 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 30% and 39%, respectively.

**Figure 14.** Compressive strength of rubber-added mortar samples with addition of untreated rubber at 5% and 7.5% replacement, aged 7, 14, and 28 days: (**a**) cement C1; (**b**) cement C2.

The results show very significant differences between the strengths obtained at 28 days with C1 and C2 when ELT rubber is added (untreated and with R1 treatment). In a similar way to the previous case, when untreated ELT is added, the design strength is only fulfil with 5% replacement, in this occasion with sizes T1 and T2. For 7.5% replacement, only the average result with size T1 reaches the design strength, but not all the individual samples, as it is shown by the standard deviation. The results evidence a trend of better technical performance associated with T1 size. However, other relevant factors allow considering the T1 size for the last stage of the study. In effect, considering the characteristics of the

ELT rubber, the largest size (T1) is easier to handle, less expensive to produce, and requires less energy to manufacture it.

**Figure 15.** Compressive strength of rubber-added mortar samples under treatment R1 at 5% and 7.5% replacement, after 7, 14, and 28 days: (**a**) cement C1; (**b**) cement C2.

Tapia [31] analysed the composition of three brands of cement, including the two used in this work. Similar results were found in that study in terms of the difference in compressive strength when comparing control mortars made with both cements (C1 and C2). This difference can be attributed to the direct relationship between concrete strength and the presence of dicalcium silicate (C2S) and tricalcium silicate (C3S) in the cement composition, which in the case of cement C1 is greater than in cement C2. Another reason found is the presence of fly ash in cement C1, which is not present in cement C2 [39] and which, in the long term, causes concretes with this compound to continue acquiring strength.

The lower reduction in compressive strength observed in rubber-based mortars with cement C1 can be explained by the presence of fly ash. Indeed, this component has been used in other investigations to improve the interaction between the rubber and the cementitious matrix, showing adequate efficacy in mitigating the reduction of concrete strength [40–42].

For these reasons, C1 cement was chosen to continue with the last stage of the study.

#### *3.3. Stage 3*

#### 3.3.1. Workability

Figure 16 shows the cone slump of the rubber-added mortar samples under the three treatments applied. The figure shows as well, the results with untreated rubber, under the same conditions, i.e., 5% substitution rate, grain size T1 (2.36–4.75 mm) and use of cement C1.

With respect to the control mortar, treatment 2 (oxidation–sulphonation) and treatment 3 (hydrogen peroxide), show slightly greater cone slumps, with the greatest corresponding to treatment R2. However, these slump values are always lower than the results of the mortars with untreated rubber. Furthermore, in all cases, the results are within the design range [39].

**Figure 16.** Cone slumps of cement mortars with 5% replacement rubber, grain size T1, using cement C1.

#### 3.3.2. Density

Figure 17 shows the densities of mortars with rubber addition under the three treatments used, and furthermore, shows the results with untreated rubber, under the same conditions, i.e., 5% substitution rate, grain size T1 (2.36–4.75 mm) and use of cement C1.

**Figure 17.** Density of mortars with addition of rubber at 5% replacement, grain size T1, using cement C1. ns = non-significant.

With respect to the control mortar, the samples present slightly lower densities due to the lower density of the rubber used, but when compared to the samples containing untreated rubber, they are higher, which is attributable to a lower presence of air in the mix, which in turn is due to the treatments that reduce the hydrophobicity of the rubber. This effect is mostly visible in the mortars under treatment R2 (oxidation–sulphonation), where the density decrease is insignificant compared to control mortar.

Using one-way analysis of variance (ANOVA) with Dunnett's post hoc, it is possible to observe that there is no statistically significant difference (ns) between the samples containing ELT, either untreated or with any of the three treatments, compared to the control mortar.

#### 3.3.3. Flexural Strength

The results of the flexural strength tests for the rubber added samples under the three treatments are presented in Figure 18. This figure shows as well, the results with untreated rubber, under the same conditions, i.e., 5% substitution rate, grain size T1 (2.36–4.75 mm), and use of cement C1.

**Figure 18.** Flexural strength of mortars with addition of rubber at 5%, grain size T1, using cement C1, after 28 days. \* (*p* < 0.05).

When comparing the treatments with untreated rubber, both treatments, R1 (hydration), and treatment R2 (oxidation–sulphonation), present mortars with better results. Only in the case of treatment R3 (contact with hydrogen peroxide) the strength of the mortars is lower. Compared to control mortar, in all cases the flexural strength is lower, decreasing by 21% in the case of mortar with untreated rubber, 17% for rubber with treatment R1, 17% for rubber with treatment R2, and 24% in the case of rubber with treatment R3.

Through one-way ANOVA, it was determined that there is a statistically significant difference between the control mortar and the samples containing ELT rubber—untreated or under any of the three treatments. Statistical significance is designated with \*, *p* < 0.05.

#### 3.3.4. Compressive Strength

The results of the compressive strength tests for the rubber added samples under three treatments are presented in Figure 19. Furthermore, the figure shows the results with untreated rubber, under the same conditions, i.e., 5% substitution rate, grain size T1 (2.36–4.75 mm), and cement C1.

**Figure 19.** Compressive strength of mortars with 5% replacement rubber addition, grain size T1, using cement C1, after 28 days. \*\*\* (*p* < 0.001); \*\*\*\* (*p* < 0.0001).

Compared to the control mortar, in all cases, the compressive strength is lower, with a loss of 29% in the case of mortars with untreated rubber, 24% when the rubber is under treatment R1 (hydration), 24% under treatment R2 (oxidation–sulphonation), and 26% in the case of treatment R3 (contact with hydrogen peroxide). While it is true that the average result of the untreated ELT samples fulfil the design strength, which is not necessarily valid for all samples as it is shown by the standard deviation. On the contrary, when the ELT rubber is treated, the design strength of 350 (kg/cm2) is always fulfilled, including the standard error. This is valid for the three treatments, but with better results for treatments R1 and R2, similar to the case of flexural strength.

Using one-way analysis of variance (ANOVA) with Dunnett's post hoc, it is possible to visualize that there is a statistically significant difference between the results of the control samples and the ones with ELT rubber, either untreated or with any of the three treatments. However, in terms of statistical significance, those differences are less for treatments R1 and R2 (\*\*\* *p* < 0.001) than for treatment R3 and the untreated samples (\*\*\*\* *p* < 0.0001).

In addition to the technical performance related with the compressive strength, it 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.

3.3.5. Contact Angle

• Treatment R1

The contact angle tests were performed in order to observe the variation of the angle formed by the water on the modified and original rubber particle; thus, evaluating the hydrophilicity of the rubber surface by means of the angle value. Figure 20 shows that the unmodified rubber and the rubber modified with the first treatment have an angle value of 161.4◦, and 131.3◦, respectively, demonstrating that the adhesion between the rubber and the water increases slightly.

**Figure 20.** Contact angle: (**a**) unmodified rubber 161.4◦, (**b**) rubber under treatment R1, 131.3◦.

• Treatment R2

Figure 21 shows the contact angle of the rubber under the second treatment. There is a lower contact angle value compared to the unmodified rubber because the adhesion forces are larger compared to the cohesive forces. This results in the liquid being attracted to the solid and spread out; therefore, it is possible to conclude that this method increases the hydrophilic character of the ELT rubber surface.

**Figure 21.** Rubber contact angle under the second treatment (R2), 120.3◦.

• Treatment R3

Figure 22 shows that the rubber modified with the third treatment has an angle value of 145.9◦. Therefore, it has a low wettability due to the fact that the forces of attraction are lower and the surface tends to repel the liquid. The contact angle value is also affected by the surface roughness, as the rubber particle has small protuberances and the water droplet rests between their peaks, resulting in less contact and, thus, a larger contact angle.

**Figure 22.** Rubber contact angle under the third treatment (R3), 145.9◦.

3.3.6. Scanning Electron Microscope (SEM) Analysis

• Treatment R1

Using the SEM technique, it was possible to obtain images of the surface of the rubber hydrated with water (Figure 23). Differences in the surface can be observed, as the surface of the unmodified rubber is rough and has many pores, while the surface of the modified rubber is smooth and less porous.

**Figure 23.** Rubber surface: (**a**) unmodified, (**b**) under treatment R1.
