## 3 Results and Discussion
### Forced Migration Test  
In order to assess the influence of the addition of red mud in the durability of the mortars, the non-steady-state chloride migration coefficient D\({}_{\mathrm{NTB}}\) has been determined. The changes with time of this parameter for the analysed mortars can be observed in Figure 10.  
The mortars with red mud showed similar or even lower chloride migration coefficients overall than those observed for CEM I and CEM II specimens at the majority of studied ages. At seven hardening days, the coefficient D\({}_{\mathrm{NTB}}\) was higher for CEM II and RM5 mortars than for the rest of the analysed specimens. However, as shown in Figure 10, this coefficient showed a clear decreasing tendency for all the studied mortars overall. It is interesting to note that at 45 hardening days, the majority of samples with red mud presented smaller D\({}_{\mathrm{NTB}}\) coefficients in comparison with CEM I, with the exception of RM5 specimens, whose chloride migration coefficient was practically equal to that noted for CEM I samples, but lower than that corresponding to CEM II. Lastly, after 90 hardening days, the D\({}_{\mathrm{NTB}}\) coefficient values for all the red mud mortars showed practically no differences with  
Figure 9: Results of impedance spectroscopy capacitance C\({}_{2}\) for CEM I, CEM II, RM5 and RM20 mortars.  
the D\({}_{\rm NTB}\) value obtained for CEM I, although they were greater than the migration coefficient observed for CEM II samples.  
The results for the chloride migration coefficient are generally in agreement with the results of the microstructural study. The refinement of the pore network observed for the red mud mortars would mean that it would be more difficult for chlorides and other aggressive substances to ingress through these materials. It is very significant that despite replacing clinker with red mud, the value of the chloride migration coefficient had similar values for all the samples tested after 90 days. The cement that contained fly ash gave lower values for the migration coefficient, which would mean that it would be more resistant to chloride ingress [54; 55], which can be partly attributed to the high chloride binding capacity of fly ash [56]. Despite reports that red mud also has a slight chloride binding capacity [23; 27], it seems that this addition is not as efficient as fly ash. However, it provides the same behaviour related to chloride ingress resistance than the ordinary Portland cement, at least in the short-term, with an added value from an environmental point of view produced by the reutilization of a pollutant. Regarding the environmental benefits of using cement-based materials with red mud, the environmental assessment performed by [PERSON] et al. [31] showed that when red mud content increases from 0% to 25%, CO\({}_{2}\)-equivalent, as a measure of global warming potential, decreases from 556.8 Kg m\({}^{-3}\) to 409.9 Kg m\({}^{-3}\), implying that cement-based material sustainability increases with an increase of red mud content. Furthermore, those authors [31] also pointed out that with an increase in red mud content from 0% to 25%, the reduction in the quantities of CO, NO\({}_{\rm X}\), Pb and SO\({}_{2}\) is about 32.5%, 31.8%, 17.1% and 22.4% respectively. Finally, the relatively good behaviour of red mud regarding the resistance to chloride ingress observed here will be in line with other investigations [23; 24; 25; 26; 27], in which it has been shown that red mud has good performance in relation to the protection against corrosion of steel reinforcements embedded in concrete.