## 1 Introduction  
In recent years, efforts to reduce greenhouse gas (GHG) emissions have continued worldwide. In Korea, the government has established a roadmap for GHG reduction based on prescribed target values to be fulfilled [1]. In the construction industry, Portland cement is widely used to produce ready-mixed concrete and precast concrete products. However, a large amount of CO\({}_{2}\) is generated during the production of Portland cement, and various studies are being conducted to reduce the amount of cement and enable the use of cement substitutes [2, 3, 4, 5]. Among industrial by-products, fly ash generated in thermal power plants has a pozzolanic reaction and is widely used in concrete. Specifically, in Korea, approximately 10 to 20% of fly ash is substituted for cement content in ready-mixed concrete [6]. In addition, various steel industry by-products are widely used in the construction industry. Ferronickel slag is a by-product generated by melting materials such as nickel ore and bituminous coal, which are used as raw materials to produce ferronickel at high temperatures. More than 2 million tons are yearly generated in South Korea [7]. To use ferronickel slag in the construction industry, several guidelines and standards were established [8, 9, 10], but the recycling rate is still insignificant, and ferronickel slag that has not been recycled is buried and neglected in yards, causing environmental pollution [11]. As an effort to increase the recyclability and value of ferronickel slag, some studies proposed to convert ferroinckel slag into fine powder and use it as a cement substitute [12, 13, 14].  
[PERSON] et al. [12] reported that fine ferroinckel-slag powder can fill the voids in paste; the higher the replacement rate of this fine ferroinckel-slag powder, the lower the chloride ion penetration. According to a study by [PERSON] et al. [11], ferronickel slag is effective in reducing the hydration heat and drying shrinkage of cement mortar when ferronickel-slag powder is used as a cement substitute.  
In addition, in a study by [PERSON] et al. [13], it was reported that the mixing of ferronickel-slag powder contributes to achieve tight pore structures in the cement matrix.  
Interestingly, it was also reported that the drying shrinkage of cement mortar is reduced when used with other mineral admixtures, in contrast with using ferronickel-slag powder alone [14].  
Although a number of studies focused on recycling ferronickel slag, few addressed the use of ferronickel-slag powder and fly ash together. The combination of ferronickel slag and fly ash to achieve good engineering performance of cement mortar is expected to contribute to the increase in recycling of both materials. This study evaluates the compatibility between ferronickel-slag powder (FN) and fly ash (FA) when FA is mixed with FN at a certain ratio to increase the recycling rate of FN, which remains insignificant compared with other steel industry byproducts. The ultimate goal is to reduce environmental pollution by increasing the recycling rate of steel industry byproducts. In this study, we investigated the fluidity, microhydration heat, compressive strength, drying shrinkage, and carbonation characteristics of a ternary cement mortar using ferronickel-slag powder and fly ash as part of a study to increase the recycling of ferronickel slag.