## 3 Results and Discussion
### Mechanical Behavior and Total Porosity  
Figure 3 shows the results obtained after the hydration process for each of the evaluated systems and a comparison with the reference system (CEM I) at different curing ages (1, 7 and 28 days). In the case of CEM I (Figure 3a), it is observed how, as the hydration times increase, the compression also strength increases. This is due to the hydration process of the cement and the progressive formation of the C-S-H gel as well as secondary reaction products, such as portlandite or ettringite, among others, achieving final strengths at 28 days of curing around 45 MPa [26; 27]. Figure 3b shows the mechanical behavior of alkaline-activated slag systems activated with NaOH and sodium silicate (waterglass). As with the CEM I system, as the curing time increases, the mechanical strength also increases regardless of the activator used. However, there is a clear difference between the use of NaOH and sodium silicate. By using an activator with silicon in its composition, a much more compact and cohesive C-A-S-H gel is produced, promoting very high resistance [8; 28]. At the age of 28 days, compression strength values greater than 100 MPa are reached. On the other hand, regarding the activation of the slag with NaOH, it is possible to observe that the results at 28 days are much lower than those obtained with the use of sodium silicate (compression strengths of around 30 MPa), presenting, therefore, a difference of around 70% between both alkaline activators. Regarding the manufactured hybrid systems, Figure 3c shows the mechanical behavior of the hybrid slag and the hybrid fly ash mortars, all of which were prepared with 3-5% Na\({}_{2}\)SO\({}_{4}\) as an alkaline source. At short curing ages, it is observed that the hybrid cement with fly ash presents slightly higher strengths, but, as the curing time increases, the hybrid systems made of slag show better mechanical behavior. This is due to the fact that, in the case of slag, as hydration takes place at room temperature, the calcium present in the composition begins to dissolve and reacts with silicon to form the main  
Figure 3: Compression strengths of mortars at different curing ages (1, 7 and 28 days): (**a**) Portland cement (CEM I); (**b**) alkaline-activated slag with 4M NaOH (AAS-N) and with sodium silicate solution (AAS-WG); (**c**) hybrids cements with blast furnace slag (HS-3% and HS-5%) and with fly ash (HFA-3% and HFA-5%); (**d**) hybrid cements with porous waste (HWT) and with porcelain waste (HPT). reaction product, the C-A-S-H gel. However, since no activating solutions are added in this case, these gels are less rich in silicon and, therefore, lower mechanical strengths are obtained than those obtained for alkali-activated cements (Figure 3b). In addition, for the hybrid systems composed of fly ash, the results are noticeably inferior because, unlike slag, the ash needs a temperature input in the system to dissolve and generate the zeolites, which are the precursors of the main product reaction in the activation of the ashes (N-A-S-H gel). Regarding the addition of different percentages of Na\({}_{2}\)SO\({}_{4}\), mortars composed of 5% Na\({}_{2}\)SO\({}_{4}\) obtain slightly higher mechanical performance at longer curing times. At the age of 28 days, for the hybrid slag systems at 5% Na\({}_{2}\)SO\({}_{4}\) resistance, values are obtained around 42 MPa, whereas the hybrid ash systems at 5% Na\({}_{2}\)SO\({}_{4}\) presented results of 30 MPa at 28 days of curing. Finally, Figure 3d shows the results of the hybrid systems manufactured with the ceramic industry wastes (HWT and HPT). The results show that the compression strength is lower than all the systems previously presented. The resistance increases as the curing time increases but up to a limit since, at the age of 28 days, it is observed that they are very similar to those obtained after seven days of hydration. This behavior is due to the fact that both ceramic residues present a composition with a low CaO content similar to that of fly ash.