Figure 3 shows diffractograms of the HB and CE (OPC/GBFS 20%/80%) pastes after 28 days of curing. The diffractograms showed the presence of some crystalline components of the raw materials (Figure 1) and of the reaction products. The presence of quartz, calcite, aragonite and C-S-H can be observed in both types of concrete, and in particular, hydrated gehlenite (C₂ASH₈/C₄AH₁₃) is found in the hybrid samples; this is a hydration product characteristic of slags that are alkali activated using sodium silicate and sodium hydroxide (20) and is associated with the absence of portlandite in these samples (21). Some researchers have noted that due to the reduced amounts of OPC in the mixtures (20%), it is possible for portlandite (Ca(OH)₂) not to be found (22, 23); conversely, other studies have shown that it is possible for a small amount to participate in the reaction with the GBFS or the silicate present as an activator to generate more C-S-H (24, 25).

Table 3 shows the physical-mechanical properties of the CE and HB samples evaluated and compares them to those of a 100% OPC concrete. It is observed that the compressive strength of the CE after 28 days of curing is approximately 50% less than that of the HB and 100% OPC concretes. This behaviour is in agreement with the results of different studies that have shown that the strengths of concretes with high contents (30% - 90%) of blast furnace slag replacing Portland cement after 28 days of curing are lower than those of the 100% OPC control samples, which is due to the relatively low reaction rate of the slag (26-35). Therefore, it is concluded that gradually increasing the amount of slag in the concrete causes a reduction on the compressive strength at early ages. Regarding absorption and porosity, note that the differences in the total absorption and the porosity are directly related to the reported strength. However, the capillary absorption coefficient (K), resistance to water penetration (m) and effective porosity indicate that both the CE and the alkali-activated HB are less permeable than the 100% OPC concrete; this is attributable to greater refinement in the pore structure, behaviour that coincides with the research carried out by Rodríguez et al. (36), these results indicate the presence of a more dense and resistant structure.

Figure 4 shows the progress of the carbonation front of the HB and CE samples, which is compared with the behaviour of a 100% OPC concrete produced with the same mixture proportions. The samples were subjected to accelerated carbonation conditions (65% RH, 1% CO₂ and 25°C) after 28 days of curing.

In general, all the concretes show gradual carbonation (Figure 5 through Figure 7); however, the HB and CE samples became completely carbonated in a shorter period (99 days) compared to the 100% OPC concrete, which showed a carbonation depth of 13 mm at same age of exposure. Therefore, the high susceptibility to accelerated carbonation of the HB and CE concretes has been demonstrated, and it should be noted that although the HB sample generally has a greater compressive strength and lower permeability than the 100% OPC and CE concretes, its susceptibility to carbonation is higher.

Some authors (7, 9, 37, 38) have suggested that the greater susceptibility of the carbonation process is proportional to the amount of blast furnace slag and independent of the decreased porosity caused by the added material, especially in concretes containing 70% or more of GBFS, although their compressive strength remains high. This greater susceptibility is also associated with the decalcification of the C-S-H gel (11, 15, 39), which affects the mechanical properties, as shown in Table 4, in which the mechanical strengths of the concretes are compared before and after the samples are subjected to the accelerated carbonation process; it can be seen that after exposure to CO₂ for 99 days, compared to those samples cured in the absence of CO₂, the compressive strength of these samples decreases drastically. The residual strength of CE and HB, after 99 days of accelerated exposure to CO2, was similar (3.35 and 3.60 MPa, respectively). However, it is to be noted that due to the higher initial resistance of HB (34.04 MPa), which is approximately two times higher than the corresponding from CE to 28 days of normal curing, the loss of compressive strength calculated was superior (89%). On the contrary, in the case of OPC concrete, the compressive strength to the same age of exposure presented an increase, which is related to the lower carbonation depth (Figure 4). Similar behaviours have been found by Backharev et al., Bernal et al. (17, 40, 41), who noted that concretes containing alkali-activated GBFS exhibited higher rates of carbonation even though they are more alkaline than 100% OPC concretes and blended concretes with pozzolan. The higher alkalinity of alkali-activated concretes is due to the activators, in this case, to a mixture of sodium silicate and sodium hydroxide (with a pH of greater than 13.5), whereas the pH of the OPC paste is between 12.6 and 13.5 (11, 42). Some authors (16, 37, 43, 44) have noted that the adverse behaviour of the alkaline-activated concretes towards carbonation can be controlled and reduced by using a larger proportion of cementitious material, a lower water/cementitious material ratio and a higher Ms.

Due to the large number of factors that affect the results, it is difficult to establish a correlation between the exposure times under accelerated and natural conditions. Several authors (45–48) have suggested that there is a relationship between natural exposure (K_N) and exposure to accelerated conditions (Kc) that can be expressed using [1]

where C represents the CO₂ concentration of the accelerated environment and N represents the CO₂ concentration of the natural environment; for the purpose of analysing this condition, an environment with a CO₂ concentration of 0.1% or [N=0.1] was assumed, and the values of K^N were calculated (Table 5). In this case, C corresponds to the value used in the accelerated environment (1% CO₂) and K^C to the value obtained during the test. It should be noted that these expressions have been developed for Portland cement concrete.

Using the values of K^N found for each of the concretes evaluated, the time required for complete carbonation (t^C) can be estimated in an environment containing 0.1% CO₂, as shown in Table 5. Note that the time required for the 100% OPC concrete is greater than 50 years, which is the estimated time required for a medium-strength Portland cement concrete, whereas the HB and CE concretes require only 10 years; this demonstrates the increased susceptibility to carbonation of Portland concretes with high percentages of GBFS and hybrid alkali-activated concretes based on 80% GBFS in urban and industrial environments containing CO₂ emissions at concentrations of 0.1%. However, as explained by Duffó et al. (49), for a good concrete, the values of K are between 0.25 and 1; based on this and as Table 5 shows, the HB and the concrete CE (80% slag-20% cement) can be considered also good, as is the 100% OPC concrete subjected to natural carbonation.

Figure 8 shows the half-cell potential (Ecorr) over time for the steel embedded in the HB and CE concretes in an accelerated carbonation environment (65% RH, 1% CO₂ and 25°C). In general, both concretes exhibit similar behaviour. In accordance with ASTM C876 [18] and (50), during the first 95 days of exposure, Ecorr is in a zone in which corrosion may or may not occur (between -0.10 and -0.25 V vs. Ag/AgCl). After 120 days of exposure, the values of Ecorr for the concretes decrease drastically until they reach the level at which the probability of corrosion is 90%. These results coincide with those obtained in the carbonation front test, which showed complete carbonation of the concretes at the same exposure age (Figures 4 through Figure 7). After this age, Ecorr values of both concretes, hybrid and reference, show a variable behaviour, with potential values between -0.25 and -0.60 V vs. Ag / AgCl until approximately 700 days in an accelerated carbonation chamber, thus remaining in the zone of 90% probability that corrosion will occur. It is observed, from the 425 days of exposure in samples of reinforced hybrid concrete submerged in water, as well as in those exposed to accelerated carbonation, a similar behaviour regarding the corrosion potential.

Figure 9 shows the values of the corrosion current density (icorr) of the steel embedded in the concretes evaluated, calculated from measurements of LPR using the Stern-Geary equation. All the concretes showed high icorr above 2 μA/cm². These results are strongly correlated with those obtained in the corrosion potential test (Figure 8), in which both concretes are likely to become corroded. The steels extracted from the different concretes to confirm that the corrosion has occurred (Figure 10). The results of the electrochemical tests are associated with the results of Aperador et al. (51,52). These authors evaluated alkali-activated concretes containing GBFS (100%) and attributed the high carbonation rate, among other factors, to micro-cracks produced by the contraction of these materials during the drying process. These micro-cracks facilitate the entry of CO₂ into the structure; this has also been explained by other researchers (14, 39). Additionally, Bernal et al. (53) noted that the advancement of the carbonation process contributes to increases in the porosity, and consequently, this phenomenon contributes to the acceleration of the corrosion process. Alcaide et al. (54) studied the effect of carbon fibre on alkali-activated slag mortars reinforcing steel corrosion and showed that the maximum corrosion density reached in the carbonation process is 5.7 μA/cm², after around 50 days this value flattened to 2.5 μA/cm², this is because to the interaction between the CO₂ and C-S-H gel and additionally to the presence of carbonaceous materials which modify the reaction kinetics. However, it is possible to increase the useful life of these alkali-activated reinforced concretes and to decrease their susceptibility to carbonation by appropriately controlling the design of the concrete, especially the proportion of cementitious material and the type and proportion of the alkaline activator (55–57).