## 2 Experimental Setup
### Impedance Spectroscopy  
The changes in the pore network of the mortars have also been followed with the non-destructive impedance spectroscopy technique. It has several advantages compared to other classical techniques, such as the ability to get global data of the microstructure of the samples [44; 45]. Recently, several works have been published [14; 38; 46] where this non-destructive and global technique has been used for analysing the evolution of the pore structure of different cement-based materials (pastes, mortars and concretes) with additions, obtaining very good results. Some of these additions are well-known and widely used, such as slag and fly ash [35], and others are relatively new, like waste brick powder [36] and glass powder [37]. Impedance spectroscopy has also been successfully used for studying how aggressive and non-optimum environments [14; 38] affect the evolution of the microstructure of cement pastes, mortars and concretes. However, there is scarce knowledge about applying this technique for studying cementitious materials that incorporate red mud to replace clinker [26], although, in view of the previously mentioned successful studies, impedance spectroscopy could be suitable for following the microstructure evolution of materials with red mud.  
The measurements were performed using an impedance analyser Agilent 4294A manufactured by Agilent Technologies (Kobe, Japan), which permits capacitance measurements in the range from \(10^{-14}\) F to 0.1 F, having a highest resolution of \(10^{-15}\) F. In this study, the interval of frequencies for getting the impedance spectra was between 100 Hz and 100 MHz. The electrodes used for measuring had a circular shape (8 cm diameter) and they consisted of a copper piece with the same diameter to which a flexible graphite sheet was attached. For performing the measurements, contacting and non-contacting methods were used [44]. In Figure 1, the experimental setup of the impedance measurement of a mortar sample using the contacting method is shown. The impedance spectra, after Kramers-Kronig validation [47], were fitted to the two time constants equivalent circuits established by [PERSON] et al. [44] (see Figure 2). These circuits are formed by resistances and capacitances, which give data related to different characteristics of the microstructure of cement-based materials.  
In this study, only the resistance R1 and the capacitance C2 were analysed. The impedance resistance R1 provides data associated with the percolating pores of the specimen and the capacitance C2 provides data about the pore surface in contact with the electrolyte which fills the pore structure of the sample. The impedance parameter C2 is present in the equivalent circuits of both contacting and non-contacting methods (see Figure 2), and in this research, only its values obtained with the non-contacting method have been analysed, due to its better accuracy.  
The evolution of the resistance R1 and the capacitance C2 was studied for the mortars with the lowest and the highest content of red mud (RM5 and RM20 respectively), as well as for CEM I and CEM II specimens at the hardening ages of 10, 28, 45 and 75 days. Four different disks of 1 cm thickness were tested for each one of the abovementioned binders.