3.4.2. Rebound Number

Figure 9 shows the results obtained from the rebound number tests to determine the compressive strength. It can be seen that the values obtained for the MC are similar to those presented in Figure 8: at the age of 28 days the control had an approximate value of 33 MPa for both simple compressive strength and rebound number tests. In the 7 day test, the ETC concretes reported compressive strength values lower than the MC by higher percentages than those reported in Figure 8, while over time these values increased in the five ETC mixtures, with this behavior likely being due to the effect of using materials with pozzolanic characteristics such as SCBA and SF. Unlike the compressive strength test using the cylinder, the values reported using the sclerometer for the five ETC concrete mixtures at the ages of 28, 90 and 180 days were always lower for the five ETC mixtures than for the MC; however, the mixtures with the best performances were still ETC20 and ETC30, which confirms the behavior in the compression test reported in Figure 8, where it was shown ETC20 and ETC30 were the best ETC mixtures at 90 and 180 days. The results obtained with the non-destructive rebound number test coincide with those reported in the literature on the use of said test to approximate the mechanical resistance of concrete elements in situ or in the laboratory, as reference values of resistance which must be supported by compressive strength tests of the evaluated concretes [78,79].

**Figure 9.** Compressive strength according to the rebound number.

3.4.3. Electrical Resistivity of Concrete Mixtures

Figure 10 shows the results obtained from the Electrical Resistivity test of the six concrete mixtures in this study. The tests were carried out at the ages of 7, 14, 28, 90 and 180 days. In Table 4 it can be seen that at the age of 7 days, almost all mixtures were found to have high corrosion risk, with values less than 10 kΩ-cm for the specimens of the MC, ETC-10, ETC-20, ETC-30 and ETC-40, mixtures. These values coincide with findings from the literature, where at early ages resistivity values are lower [80]. The only specimen that presented a higher value was the ETC50 mixture, reporting a resistivity of 13.45 kΩ-cm after 7 days, which indicates a moderate risk of corrosion. At the age of 14 days, all concretes showed a minimal increase in Electrical Resistivity, but both the ETC-40 and ETC-50 mixtures presented electrical resistivity values greater than 10 kΩ-cm, representing a moderate corrosion risk. At the age of 28 days, the benefit of the combination of the SCBA and the SF as pozzolanic materials was observed, as the durability of the ETC-20, ETC-30, ETC-40 and ETC-50 mixtures increased, with increases in the electrical resistivity values correlating with the percentage of substitution, with values that placed all of them in the moderate corrosion risk zone. ETC-20 and ETC-30 possessed electrical resistivities of 13.43 and 20.03 kΩ-cm while ETC-40 and ETC-50 presented the best performance with values of 47.5 and 51.4 kΩ-cm respectively. This is in agreement with the results of Bagheri et al., who evaluated concretes with different percentages of substitution of Portland cement with FA and SF, and found that the concretes with 20% and 30% FA and SF possessed electrical resistivity values at 28 days two times greater than that of the control mix [81]. At the age of 90 days, all Ecological Ternary Concretes (ETC-10, ETC-20, ETC-30, ETC-40, ETC-50ETC) reached the stage of moderate corrosion risk. The ETC-40 and ETC-50 concretes continued to present the best performances, with electrical resistivity values of 179.56 and 170.24 kΩ-cm respectively.

Finally, at the age of 180 days, the concretes that presented low electrical resistivity were the control mixture MC with a value of 10.88 kΩ-cm, followed by the concretes ETC-10 and ETC-20 with values of 12.74 and 54.39 kΩ-cm respectively. The concretes that presented the best performances were the ETC-30, ETC-40 and ETC-50 specimens, with values of 143.53, 191.44 and 156.20 kΩ-cm respectively. As can be seen, the Ecofriendly Ternary Concrete with 40% substitution of the SCBA-SF combination for Portland cement, mixture ETC-40, showed the best performance; this increase in electrical resistivity agrees with the results of Sadrmomtazi et al. [82], showing that including silica fume has positive effects on the fiber–matrix transition zone structure while increasing mechanical strength and specific electrical resistivity by up to 20 times compared to controls, due to the production of pozzolanic reactions and decreased concentration of portlandite, which increases uniformity and density as well as bond quality. It is observed that all ETC mixtures performed better in the electrical resistance test compared to the compression resistance test, and this behavior coincides with a report in the literature and is associated with the fact that the total volume of concrete pores is not reduced by pozzolanic reactions, but the pore structure becomes more discrete [83].

**Figure 10.** Electrical resistivity of the study concretes.

#### **4. Conclusions**

In all ETC mixtures there was a decrease in workability, which is attributed to the demand or absorption in excess of water due to pozzolanic materials. However, there were no significant variations in temperature or unit weight in the fresh state compared to the control mixture. The tested slump, temperature and unit weight of the ETC mixtures in their fresh state met requirements for the construction of civil works, such as bridges, pavements, buildings, dams, etc.

The results of compressive strength at 180 days indicated that the optimal percentage of substitution of CPC by combination of SCBA-SF was 30% followed by 20%, due to their increases in compressive strength of 7.13 and 5.58% respectively compared with the MC. The ETC-40 mixture also presented a compression resistance equal to that of the MC.

The rebound number test is a non-destructive test that can be used to evaluate the compressive strength of ETC concretes in the laboratory and on site, with the reservation that they are not considered as definitive values but rather as approximations, and it is always recommended to supplement rebound number tests with simple compression tests on cylinders and cubes.

All ETC mixtures presented better results in the electrical resistance test compared to the compression resistance test, suggesting that the ETC concretes were more durable and had a higher resistance to corrosion compared to the control mixture.

The Eco-friendly Ternary Concrete with 50% substitution of SCBA-SF (ETC-50) displayed a resistance of 20.09 MPa at 180 days, sufficient for the construction of minor works.

The use of ETC concretes has a very significant sustainability impact by contributing to the reduction of CO2 emissions caused by Portland cement, replacing up to 50% of it with SCBA and SF waste and generating a culture of recycling in countries such as Mexico for the use of waste that, like SCBA, has lacked a defined use and previously been discarded as garbage.

**Author Contributions:** Conceptualization, M.A.B.-Z., R.C., G.S.-H. and V.M.M.-L.; Methodology, L.L.-R., A.L.-G., J.M.M.-R., H.A.-F., A.L.-S., C.T.M.-R., M.A.B.-Z. and R.C.; Data Curation, L.L.-R., J.M.M.-R., M.A.B.-Z., R.C., G.S.-H. and V.M.M.-L.; Writing—Review and Editing, L.L.-R., M.A.B.-Z., R.C., G.S.-H. and V.M.M.-L.; Visualization: M.A.B.-Z., R.C., G.S.-H. and V.M.M.-L.; Supervision: M.A.B.-Z., R.C., G.S.-H. and V.M.M.-L.; Funding acquisition: M.A.B.-Z. and R.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** M.A. Baltazar-Zamora, et al., thank PRODEP for the support granted by the SEP to the Academic Body UV-CA-458 "Sustainability and Durability of Materials for Civil Infrastructure", within the framework of the 2018 Call for the Strengthening of Academic Bodies with IDCA 28593.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The results are original of the research work.

**Acknowledgments:** M.A. Baltazar-Zamora, et al., thank PRODEP for the support granted by the SEP to the Academic Body UV-CA-458 "Sustainability and Durability of Materials for Civil Infrastructure", within the framework of the 2018 Call for the Strengthening of Academic Bodies with IDCA 28593. The authors thank Brenda Paola Baltazar García for technical support.

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