## 2 Materials and Methods
### Experimental Methods  
#### 2.4.1 Chemical Characterizations  
Three methods of chemical characterization were employed to reach a comprehensive understanding of the chemical processes involved in the combined sulfate-chloride attack. Thermogravimetric analysis (TGA) was primarily performed to quantify the presence of portlandite by observing sample mass variations with respect to temperature variation. The measurements were conducted using a NETZSCH STA449 F1 instrument in an inert nitrogen environment, with a temperature range from 25 to 1250 \({}^{\circ}\)C and a heating rate of 10 \({}^{\circ}\)C per minute. The derivative of the thermogravimetric analysis curve (DTG) was obtained to identify significant mass variations, such as losses or gains.  
Fourier Transform Infrared spectroscopy (FTIR) was also employed to characterize the chemical composition of the cementitious materials, qualitatively identifying chemical bonds present in the materials. Measurements were obtained using a Nicolet iS50 spectrometer, covering a spectral range of 400 to 4000 cm\({}^{-1}\). The obtained bands were compared to those found in the literature.  
Furthermore, Raman spectroscopy was utilized to evaluate the depletion of sulfate ions in the solutions. The Raman spectrometer used was an iRaman by BWTek, equipped with a 50 mW output 532 nm laser. The instrument operated in the spectral range of 150-4000 cm\({}^{-1}\), with a spectral resolution of 4 cm\({}^{-1}\). It was equipped with an immersion probe for analyzing the solutions and monitoring the consumption of sulfate ions during various stages of the chemical attack. The integration time was set at 15 s, and each obtained spectrum was the average of two spectra.  
These three methods were applied to both the powder samples (TGA and FTIR) and the solutions (Raman spectroscopy) after respective exposure durations of 1 (T1), 2 (T2), 5 (T3), 11 (T4), 18 (T5), and 25 days (T6). The combined use of these three methods led to a better understanding of the chemical composition of the samples. This was crucial for analyzing the chemical effects resulting from the coupled interaction of chloride and sulfate ions with a cementitious matrix.  
Additionally, the pH of the solutions was measured using a Mettler Toledo pH meter at room temperature (-22 \({}^{\circ}\)C) at the same time intervals as those chosen for the other analysis methods. The pH results were double-checked using Universal Indicator pH-indicator papers. The studies conducted by [PERSON] et al. (2018) and [PERSON] et al. (2019) have highlighted the significant impact of sulfate attacks on the pH of surrounding solutions and vice versa. During sulfate attacks on cementitious materials, sulfate ions react with calcium hydroxide (Ca(OH)\({}_{2}\)), calcium aluminate, and low-sulfate calcium sulfo-aluminate, leading to the formation of expansive compounds. This, in turn, can cause expansion and cracking in RC structures. A key finding from these studies is that the chemical reaction between sulfate and concrete generates hydroxyl ions, which subsequently increase the pH of the surrounding solution. The concentration of sulfate in the solution was observed to correlate with the liberation of hydroxyl ions. This relationship is linked to sulfates replacing the two moles of hydroxyl during their interaction with concrete, leading to an increase in pH. The interplay between sulfate attacks and pH variations is a critical aspect of understanding the degradation mechanisms in cementitious materials.  
Indeed, the study conducted by [PERSON] et al. (2019) has interpreted the influence of chloride attacks on the leaching of calcium into surrounding solutions. The presence of chloride ions can promote the release of calcium from cementitious materials, leading to its dissolution into the surrounding solution. For this, the pH of the solution was measured using a pH meter after 1, 2, 5, 11, 18, and 25 days of exposure.