## 3 Results and Discussion  
### Mercury Intrusion Porosimetry  
The total porosity results for CEM I, CEM II, RM5, RM10, RM15 and RM20 mortars can be observed in Figure 3. At seven days of age, RM10 and RM15 mortars showed lower total porosities compared to CEM I and CEM II. On the other hand, at that hardening age, this porosity is higher for the RM5 and RM20 specimens than for mortars prepared with CEM I and CEM II. The total porosity of the RM5 samples decreased with time, whereas it kept approximately constant for the rest of the mortars with different contents of red mud. After 90 hardening days, all the studied mortars showed similar values of total porosity, with the exception of RM20 specimens, whose porosity was between 2% and 3% higher. The pore size distributions obtained for the studied samples are shown in Figures 4 and 5. As can be observed in both figures, the pore network of the mortars with red mud addition is more refined than those obtained for CEM I and CEM II samples, which suggests the result that the red mud samples generally showed higher volumes of pores with diameters lower than 100 nm (addition of volumes of ranges \(<\)10 nm and 10-100 nm), compared to CEM I and CEM II specimens. In addition to this, it is important to note the relatively high volume of pores with sizes included in the ranges \(<\)10 nm and 10-100 nm observed for mortars with red mud, which increased with hardening time. Furthermore, the microstructure of the mortars became more refined with increases in red mud content. These results could be related to the possible pozzolanic activity of the red mud [21; 22], or due to the fact that the fine particles of red mud may act as filler [22; 23], occupying part of the existing pores and producing a reduction of the pore sections. On the other hand, a slight increase in the volume of the  
Figure 3: Results of total porosity for mortars prepared with contents of 5% (RM5), 10% (RM10), 15% (RM15) and 20% (RM20) of red mud, compared to those obtained for ordinary Portland cement (CEM I) and Portland cement with fly ash (CEM II) mortars. largest pore range has been observed in binders RM5 and RM20 compared to the reference mortars. Finally, the fluctuation of the intrusion volume for CEM I and CEM II samples at 45 days and 75 days could be due to a more efficient compacting of the samples. The percentages of mercury retained in the studied samples after testing are shown in Figure 6. At early hardening ages, the red mud mortars showed greater percentages of Hg retained compared to CEM I and CEM II specimens. This would suggest that the addition of red mud would entail an increase of the tortuosity of the pore network of the mortars. However, at 90 days, the Hg retained values were very similar for all the tested mortars, although it was slightly higher for those with a 5% red mud content. Figure 4: Pore size distributions obtained for CEM I, CEM II, RM5 and RM10 mortars. Figure 5: Pore size distributions noted for mortars with 15% (RM15) and 20% (RM20) red mud content. Pore size distributions of CEM I and CEM II mortars have also been included in order to facilitate the comparison between the abovementioned red mud mortars and those made using commercial cements. In view of the mercury intrusion porosimetry results, it could be pointed out that the use of red mud as a cement addition produces an improvement of the pore network of the mortars, especially in relation to its higher refinement. The presence of a high volume of pores with small diameters in red mud mortars could be interpreted as being due to the pozzolanic activity of these muds [21; 22]. This would mean that the particles of red mud could be capable of reacting with the clinker hydration products, forming calcium silicate hydrate (CSH) phases [22; 27], which would progressively fill the pores and produce a more compact microstructure.