## 3 Results and Discussion
### Mechanical Strength Test  
The results of both compressive and flexural strengths are shown in Figures 11 and 12, respectively. As described in the experimental setup section, for checking the average effects on the mechanical properties of the addition of red mud, the abovementioned strengths were only studied in the RM10 and RM15 specimens, which were compared to those obtained for CEM I and CEM II mortars. Figure 10: Results of the non-steady-state chloride migration coefficient for the studied mortars. The compressive strengths of red mud and CEM II mortars at seven days were similar (see Figure 11), although they were smaller than those observed for CEM I. At 28 days, the compressive strength noticeably increased for RM10 and CEM II mortars, showing the highest strengths of all the studied mortars at that hardening age. This rise was also observed for RM15 samples, although their compressive strength increase rate was lower. Nevertheless, since then, the compressive strength of red mud specimens started to decrease, while those for CEM I and CEM II mortars increased. After 90 hardening days, the highest compressive strength values corresponded to the CEM II mortars followed by CEM I, whereas the lowest values of this parameter were observed for the RM15 specimens. In view of the compressive strength results, it is important to point out that the value of this parameter for RM10 mortars was approximately 45 MPa at 28 days. This value is greater than the minimum value required by the Spanish and European standard UNE-EN 197-1 [39] for cements with strength class 42.5. However, due to the decrease of this parameter observed from that age, the RM10 mortars would not achieve this requirement at greater hardening times. On the other hand, the value of this strength for RM15 mortars was approximately 39 MPa at 28 days, so they would not fulfil the previously explained requirement for strength class 42.5 cements. In spite of that, this RM15  
Figure 11: Results of compressive strength for CEM I, CEM II, RM10 and RM15 mortars. Figure 12: Results of flexural strength for CEM I, CEM II, RM10 and RM15 mortars. mortar could be classified as strength class 32.5 [39], because it far exceeded the minimum strength value required for that class at 28 days and greater hardening ages, despite a slight decrease of this parameter overtime. Regarding the flexural strength (see Figure 12), the values of this parameter showed scarce differences up to 45 days for CEM II and red mud mortars, all of which were higher than those obtained for CEM I. Nevertheless, after 90 hardening days, the greatest flexural strength corresponded to CEM II specimens. At that age, this parameter was relatively similar for CEM I and red mud mortars. In view of the mechanical properties results previously described, it can be pointed out that the addition of red mud produced the development of lower compressive strengths after 90 hardening days in the mortars, compared to those made with commercial cements without red mud. This result would be in line with other studies, in which red mud has been studied in cementitious materials. [PERSON] et al. [57] reported lower mechanical strength in mortars when red mud level increases. [PERSON] et al. [22] also observed a decrease of compressive strength when the red mud content in mortars is high, pointing out that the most appropriate substitution is up to 5% of red mud from a mechanical properties point of view. Finally, [PERSON] et al. [31] studied red mud lightweight concretes and they concluded that by increasing the red mud content, the compressive strength decreased, providing the result that the 28-day compressive strength of the specimen containing 25% red mud was about 29.5% lower than that of the reference concrete.