## 1 Introduction
Despite the lack of information on the safety hazards of the use of unencapsulated EAF slag, it is safe to use in an encapsulated environment [21]. Concrete mixes containing this slag as aggregate were found to have a deeper penetration of CO2 under pressure [22], slightly higher expansion, and similar compressive strength compared with limestone aggregate concrete [23]. Compared with natural aggregates, concrete containing EAF slag aggregate has been found to have higher strength levels [24] due to the relatively porous nature of the slag particles, which allows cement to penetrate the aggregate, leading to an interlocking effect that contributes to strength [25]. As the demand for sustainable materials increases, research has been conducted worldwide on the use of EAF slag as a supplementary cementitious material [26]. Ground granulated blast furnace slag (GGBFS) is one of the three slags and has been widely used in concrete production for several years [27]. There is no significant reduction in compressive strength due to the use of EAF slag and GGBFS up to 30% in cement mixes, but there was a notable delay in their hydration reaction at the beginning of the age [28]. [PERSON], et al. [29] used EAF slag up to 20% in cement mixes and found that as the amount of slag increased, the compressive strength decreased until the end of 90 days. In both studies, it was observed that there was a high Fe2O3 content in EAF slag, which was not present at such levels in alternative binders. [PERSON], et al. [30] attempted to reduce the amount of Fe2O3 present in EAF slag before including it as a mineral additive. This was achieved by first performing an Al-dross reduction reaction, followed by a direct reduction in carbon, resulting in a final Fe2O3 content of less than 5%. The processed EAF slag was mixed with cement and exhibited physical properties comparable to those of commercialized GGBFS. If properly treated, processed EAF slag can be used in traditional cement-based materials while also meeting the necessary standards for hydraulic properties for commercial use [30]. CO2 can be stored in untreated EAF slag [22], which has not been well studied. Since EAF slag contains large amounts of calcium, it could form CaCO3 that can sequester CO2 in its microstructure [31]. Until now, other steel slags, such as ladle furnace slag, have been suggested to be more efficient in sequestering CO2 than EAF slag, but if hydrated matrices and leachates are separated, the capacity of EAF slag to sequester CO2 gas can increase [32]. Another study developed a supercritical carbonation process that involves combining EAF slag with water and processing it for 48 h with supercritical CO2 at a temperature and pressure of up to 80 \({}^{\circ}\)C and 120 bar [22]. The introduction of a dose of fine EAF slag to wollastonite mortar mixes can promote the formation of additional carbonates, resulting in better sequestration of CO2 in the concrete microstructure, and it can promote the formation of calcium silicate hydrate (CSH) with crystalline structure. In this study, the potential use of untreated EAF slag in wollastonite mortar mixes was investigated. Wollastonite and EAF slag blends have shown higher efficiencies in CO2 storage under specific CO2 pressure and temperature conditions and have also indicated superior performance. Wollastonite was substituted for EAF slag in the range of 20%, 40%, and 60% by weight to ensure reasonable levels of homogeneity between mortar samples. After curing them in CO2-rich environments for up to 7 days, their workability and compressive and flexural strengths were evaluated. The results of the X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and mercury intrusion porosimetry (MIP) studies revealed the microstructure of mortar samples cured in a CO2-rich environment. The results obtained led to a discussion of the replacement levels of EAF slag with respect to the performance of the wollastonite mixes.