2.4.3. Electrical Resistivity

Electrical resistivity tests were carried out on the six concrete mixtures MC, ETC-10, ETC-20, ETC-30, ETC-40 and ETC-50. Electrical resistivity is considered a very important physical property to determine the quality and durability of concrete [57,58]. Several investigations have shown that the level of corrosion or resistance to corrosion of reinforcing steel in concrete exposed to aggressive media can be determined by electrical resistivity [59,60].

The electrical resistivity test was carried out according to the ASTM G57-07 standard [61], according to the specified equipment requirements and procedures for the measurement of resistivity in the laboratory and on site. The DURAR Network manual [62] indicates the criteria for interpretation of the resistivity results obtained and their relationship with the risk of corrosion of the reinforced concrete, which are presented in Table 4. The tests were carried out at 7, 14, 28, 90 and 180 days. Figure 4 shows the arrangement to carry out the electrical resistivity test.

**Table 4.** Electrical resistivity in concrete and risk of corrosion [20].


**Figure 4.** Characteristics of the electrical resistivity test.

#### **3. Results and Discussion**

*3.1. Slump*

Figure 5 shows the slumps in cm of the six study mixes, the control mix (MC) and the five Eco-friendly Ternary Concrete mixtures (ETC-10, ETC-20, ETC-30, ETC-40, ETC-50ETC).

**Figure 5.** Slump of study mixtures (cm).

A decrease in workability or slump was observed in the five ETC mixtures; however, the ETC-10 mixture presented only a 7% decrease (0.5 cm) with respect to the control mixture (MC), with a value of 6.5 cm, which is considered an acceptable workability slump. With an increase to 20% in the percentage of substitution of CPC 30R with the combination of SCBA-SF, the slump showed a decrease of 50% (3.5 cm) with respect to the control mix (MC); this decrease in workability is attributed to the demand or absorption in excess of water due to pozzolanic materials [63,64], as is the case for SCBA and SF. For the ETC-30 mixture the slump was similar to that of the ETC-20 mixture, reaching a slump of 3 cm, which indicates a decrease of about 60% compared with the control mixture. In the case of the ETC-40 and ETC-50 mixtures, the effect of substituting CPC 30R by 40% and 50% respectively had a decisive effect in reducing the workability of these mixtures compared

to the control mixture, with a decrease in slump of 80% for the ETC-40 mixture and 85% for the ETC-50. This behavior is due to excess water absorption by the supplementary materials used; therefore, in several investigations where concretes with large volumes of pozzolanic materials such as blast furnace slag or fly ash were used, water-reducing or super fluidizers additives were used to obtain slumps greater than 10 cm, which allowed adequate workability of the concrete mixtures [65,66].

#### *3.2. Temperature*

Figure 6 presents the behavior of the temperatures of the six studied concretes. It is observed that five mixtures presented a temperature of 25 ◦C and the ETC-50 mixture presented a temperature of 26 ◦C. The reported temperature values are within the specifications of the ASTM C 1064/C1064M-08 standard.

**Figure 6.** Temperatures of the studied concretes in fresh states.

#### *3.3. Unit Weight*

Figure 7 presents the unit weight results of the six study mixtures. There was minimal variation between the MC mixture and the ETC mixtures, and all of the unit weight values were within the specifications for the use of concrete in structural elements of civil works according to the NMX-C-155-ONNCCE-2014 standard, which indicates that hydraulic concretes for structural use must have a normal unit weight in fresh condition between 1900 kg/m3 and 2400 kg/m<sup>3</sup> [67]. The lowest unit weight obtained was that of the ETC-50 mixture with 2149 kg/m3, with a decrease of 5% compared to the unit mass of the MC mixture; the highest unit weight was presented by the ETC-30 mixture with a value of 2288 kg/m3, 1.5% higher than the control mix. Khawaja et al. who evaluated concrete with Portland cement substitution in 5, 10, 15, 20, and 25% by SCBA, recorded an increase in unit weight of 3.13%, associated with the adhesive property of particles which reduced the concentration of induced air bubbles and consequently generated a stiffer matrix [68].

#### *3.4. Mechanical and Durability Properties*

#### 3.4.1. Compressive Strength

Figure 8 shows the compressive strength results of each of the mixtures, which were tested at the ages of 7, 14, 28, 90 and 180 days. After 7 days, the concrete ETC hadlower compressive strength values than the control mixture, of 11.32, 7.66, 30.92, 44.55 and 75.31% respectively for the ETC-10, ETC-20, ETC-30, ETC-40 and ETC-50 mixtures; this negative effect was due to the presence of alternative pozzolanic materials to cement SCBA and SF, and is in agreement with Wu et al., who showed that FA had a negative effect on strength

at early ages, but significantly enhanced the later-age strength [69]. In other studies a similar behavior has been shown even when the specimens of concrete were exposed to an aggressive medium such as sulfates [70]. At 14 days, increases in the resistance of the ETC concretes were observed, and this increase in compressive strength over time continued to 28 days, when the ETC-10 and ETC-20 concretes had 90% of the compressive strength values of the MC, with values of 28 and 29 MPa respectively, while for the ETC-30 and ETC-40 concretes the values were 22 and 23 MPa, and the ETC-50 mixture presenting the lowest compressive strength value with 13 MPa. These compressive strength values in the first 28 days coincide with the findings of various studies, where it has been shown that at 28 days sustainable or ecological concretes that substitute 20% of the CPC with supplementary materials obtain the best performance in compressive strength testing, as demonstrated by Mohamed [71], who found that a ternary concrete mixture made with the substitution of 10% FA + 10% silica fume for Portland Cement presented the highest resistance to compression in a study that covered substitutions from 10% to 50% of fly ash and silica fume for the fabrication of ternary and binary concretes exposed to different types of curing. Arif et al. found that sugar cane bagasse ash used as filler in concretes provided substantial improvements to compressive strength at substitution percentages of up to ≈20% [72]. In other studies, it has been shown that concretes with high FA contents—30%, 40% or higher—presented higher compressive strength values than the control mix, but this was due to the use of superfluidifiers and concretes with a low w/c ratio, equal to or less than 0.40 [73,74].

**Figure 7.** Unit weight of the studied concretes.

At 90 days the differences between the MC and the ETC-10, ETC-20 and ETC-30 concretes were minimal; however, lower values were observed for the specimens of the ETC-40 and ETC-50 mixtures. In percentages, the difference in compressive strength compared to the MC at 90 days was 7.22, 3.24, 1.07, 21.42 and 38.41% for the ETC-10, ETC-20, ETC-30, ETC-40, and ETC-50 mixtures respectively, with the ETC-30 mixture presenting the best performance. This result matches the findings of Le et al. [75], who concluded in their study that the compressive strength of a sample substituting OPC with 30% SCBA and 30% BFS was comparable to that of the control after 91 days [75]. At 180 days, the ETC-30, ETC-20 and ETC-40 specimens had a higher compressive strength than the specimen made with the MC control mixture; these results coincide with the literature, which indicates that at late ages the high amorphous silica content in the SCBA

reacts with the calcium hydroxide product of the cement hydration process, giving rise to the formation of additional hydrated calcium hydroxide (C-S-H), which contributes to the increase in compressive strength over time [76]. In another investigation it was found that a concrete mix made with 25% of cement replaced with processed slag, which presented the highest SiO2 content, obtained a superior compressive strength performance, reaching a value greater than 70 MPa at 90 days, which confirms the contribution to the increase in compressive strength due to pozzolanic material. A high content of SiO2 presents a high capacity to yield tobermorite (calcium hydrosilicates (C–S–H)) by reacting with portlandite (a product of concrete mineral hydration) [77]. With the results of compressive strength at 180 days, it can be concluded that the optimal percentage of substitution of CPC with a combination of SCBA-SF is 30%, followed by 20%, with increases in compressive strength of 7.13 and 5.58% respectively compared to the MC, and in third place the ETC-40 mixture, which presented a compressive strength equal to the MC. Only the mixture of Ecofriendly Ternary Concrete with 50% substitution of SCBA-SF (ETC-50) failed to develop a mechanical resistance close to that of the control mix, reaching a resistance of 20.09 MPa at 90 days.

**Figure 8.** Compressive strength of the studied concretes.
