## 1 Introduction
### Literature Review and Research Significance  
Sulfate ions penetrate the cementitious matrix and react with hydrated products. These reactions lead to the formation of ettringite and/or gypsum. These products are primarily responsible for concrete expansion and cracking. The formation of these chemical compounds induces internal pressure within the concrete matrix, leading to structural deterioration and compromising its long-term integrity [12; 13; 14]. Nevertheless, unbound chloride ions within the cement matrix, such as Friedel's salt, remain free and contribute to the corrosion of steel reinforcing bars (rebars). This corrosion not only weakens the structural integrity of the steel rebars, but also generates corrosion products that occupy a larger volume than the original steel, thereby leading to the gradual expansion and cracking of the surrounding concrete over time [15; 16]. The cumulative effects of such processes raise challenges to the durability and performance of RC structures. Presently, the individual attack mechanisms of sulfate and chloride ions are well understood [17; 18; 19; 20; 21]. However, when these ions are combined, the degradation processes and the causes of deterioration undergo significant changes due to the interaction between sulfate and chloride ions [9]. According to a study conducted by [PERSON] et al. [22], the penetration of chloride ions into the cement matrix occurs prior to that of sulfate ions. Friedel's salt is formed first during this sequence. Subsequently, sulfate ions migrate into the cement matrix and undergo a reaction with the hydrated cement products, resulting in the formation of gypsum. This process reduces the pH within the material, which, in turn, triggers the degradation of the previously formed Friedel's salt. Furthermore, the excess of sulfate ions contributes to the creation of ettringite. This sequence of events demonstrates the complex interaction between chloride and sulfate ions in the concrete environment. Indeed, a study conducted by [PERSON] et al. revealed an interesting observation regarding the interaction between sulfate and chloride ions. According to their findings, sulfate in the concrete matrix reduces chloride ions' binding capacity. As a consequence of this reduced binding, the attack by chloride ions is accelerated, which leads to an even more rapid and severe corrosion of the steel rebars within RC structures. On the one hand, some studies [23; 24; 25; 26] support the idea that sulfate leads to the destabilization of [PERSON]'s salt and reduces the binding capacity of chloride. As a result, the chloride ions' attack is accelerated. On the other hand, some other studies [27; 28] contradict this claim. They propose that sulfate ions actually attenuate chloride diffusion in concrete by decreasing its porosity. Regarding the impact of chloride on sulfate, some studies have observed that the presence of chloride ions inhibits the effect of external sulfate attack (ESA) on concrete. In contrast, the effect of chloride ions on ettringite formation is an area where research is still lacking, as noted by [PERSON] et al. [29]. The combined attack is influenced by several factors, including the porosity of the specimens, the types and concentrations of chloride and sulfate ions, including the cation type, the chemistry of the binder (characteristics of the pozzolan and its level of replacement), the duration of exposure, etc. [11; 25; 30; 31]. Furthermore, construction materials have been criticized for their substantial contribution to high carbon dioxide (CO\({}_{2}\)) emissions and natural resource depletion [32]. By the year 2010, cement alone was responsible for approximately 36% of the total 7.7 billion metric tons of CO\({}_{2}\) emissions originating from the construction industry [33]. Consequently, concrete, which heavily relies on cement as a primary component, has emerged as the second-largest contributor to global CO\({}_{2}\) emissions [34].