## 3 Results and Discussion
### Microstructure  
Clay containing water with a turbidity of around 800 NTU was passed through the concrete samples for ten repeat cycles. The SEM images in Figure 4 revealed the microstructure of the carbonated samples based on wollastonite and EAF slag. Dense formation of aragonite and calcite was observed with needle and rhombohedral morphologies in their microstructure. Tobormorite, a crystalline form of CSH, was observed in WS0 together with such carbonates, indicating the generation of CSH in wollastonite matrices made with an NaOH solution. Calcite formation was observed to be dense in WS40, which could be mainly due to CO\({}_{2}\) interactions with EAF slag. The XRD and TGA results shown in Figures 5 and 6 confirmed the presence of these carbonates in the sample matrices based on wollastonite and EAF slag. The main XRD peaks at 8.9\({}^{\circ}\), 17.8\({}^{\circ}\), 26.1\({}^{\circ}\), 26.6\({}^{\circ}\), and 29.5\({}^{\circ}\) 20 positions attribute to tobermorite, wollastonite, aragonite, quartz, and calcite crystals. Carbonation of EAF slag systems results in the major formation of tobermorite and calcite [38]. Aragonite is a form of CaCO\({}_{3}\) that can also be seen in such systems when it is allowed to dry [39]. The presence of quartz in cement-based materials containing EAF slag has been associated with greater stability and improved characteristics, including acceleration of early-age hydration and increased compressive strength [40]. Calcium aluminum oxide was detected at 35.9\({}^{\circ}\) 20 in the carbonated samples in this study and, when hydrated, it can result in the formation of ettringite, monosulfate, and calcium aluminum hydrate. However, ettringite is formed when there is enough gypsum, and ettringite is converted to monosulfate when all gypsum has been consumed in the synthesis of ettringite [41]. There is a notable increase in the amount of tobermorite formed in WS40. A similar study investigated the potential of using alkali activation to produce high-performance materials that use EAF slag. The high Al\({}_{2}\)O\({}_{3}\) and SiO\({}_{2}\) content in the EAF slag itself was found to respond optimally to the alkali activation process, and the samples demonstrated acceptable levels of bending and compressive strength [42]. The NaOH solution was used in the sample casting process of this study, which may have served as an alkali activator and interacted with Al\({}_{2}\)O\({}_{3}\) and SiO\({}_{2}\) compounds of the binders, thus contributing to the increased strength observed up to the 40% replacement level. It is also suggested that the SiO\({}_{2}\) content in the samples, as well as alkali activation, propagated the formation of an amorphous CSH, leading to a denser microstructure and, therefore, to greater strength development [42]. The EAF slag used in their study contained higher amounts of Al\({}_{2}\)O\({}_{3}\) and SiO\({}_{2}\) than the EAF slag used here, but it is possible that the wollastonite content in WS40 also contributed to the formation degree of crystalline CSH. On the contrary, studies in which alkali activation was not used observed decreases in CSH formation and compressive strength with partial replacement of cement with EAF slag at all levels of replacement, despite the addition of silica particles [43]. At higher replacement levels, such as the WS60, homogeneity cannot be achieved within the samples, as there is not enough wollastonite present to account for the relatively low amounts of silicon in the EAF slag used here and to supplement the amount of crystalline CSH products formed. Therefore, segregation and bleeding can result in decreased mechanical performance. The content of crystalline CSH has been repeatedly linked to an increase in compressive strength [43] and is supported by the observation that the WS40 has the greatest compressive strength and the highest crystalline CSH content compared with the other samples. However, the presence of CaCO\({}_{3}\) increases with the content of EAF slag and does not correlate with the strength patterns.