###### Abstract  
Cement production is the primary source of global CO\({}_{2}\) emissions in the construction industry. Blast furnace slag (BPS) has been examined as a potential substitute for cement to reduce CO\({}_{2}\) emissions. In addition, this substitution increases the long-term strength and improves the chemical resistance of mortar. However, a glassy film is formed on the surface of BFS while it is generated as a byproduct, lowering the initial strength of mortar. Notably, this film is destroyed in an alkaline environment. Thus, several studies have used solutions with various alkali activators. However, alkali activators are unsafe, as they are strong alkaline materials, and have low economic efficiency. This study experimentally improved the initial hydration reactivity of a mortar containing BFS as a substitute for cement, thereby improving its initial strength. We observed an increase in carbonation resistance. In addition, this study focused on evaluating the compressive strength and carbonation resistance of mortar prepared using BFS and alkaline water obtained from the electrolysis of a K\({}_{2}\)CO\({}_{3}\) electrolyte. Results show that alkali-activated mortar using an electrolyzed alkaline aqueous solution has higher strength and contains more hydration products than that using conventional mixing water.  
blast furnace slag; electrolysis alkaline aqueous; hydration reaction; carbonation resistance +
Footnote †: journal: Applied Sciences  
BFS is an industrial byproduct produced in steel mills. Several studies have investigated the replacement of cement with BFS to reduce CO2 emissions from cement manufacturing. This substitution is also environmentally advantageous because it involves the reuse of an industrial byproduct. Notably, the components of BFS are similar to those of cement, namely, SiO2 (33%), CaO (42%), Al2O3 (14%), MgO (6% or less), and Fe2O3 (0.5%) [3]. Furthermore, substituting cement with BFS instead of using only ordinary Portland cement (OPC) improves the long-term strength and chemical resistance of concrete [4]. However, the initial strength of concrete decreases due to the latent hydraulic property of BFS, and the resistance of the matrix to neutralization erosion weakens because the lower unit weight of cement inhibits the generation of the calcium hydroxide (Ca(OH)2) responsible for creating an alkaline environment. The glassy film interferes with the hydration reaction between BFS and water. In other words, the hydration reaction does not occur because the glassy film prevents the elution of reactive materials inside BFS. When the film is destroyed by alkaline materials, the reactive materials are eluted, leading to a hydration reaction [5; 6].  
To overcome the latent hydraulic property of BFS and improve the initial strength of mortar, previous studies have investigated the acceleration of the hydration reaction using alkali activators [7; 8; 9]. When strong alkaline substances, such as NaOH, KOH, and Na2 SiO3, exist in the BFS and fly ash, ions, such as Ca2+ and Si4+, are leached, and a hydration reaction occurs. Such substances that react with BFS and fly ash without the use of cement are called alkaline activators. However, alkali activators are expensive, and using them as construction materials may be dangerous, owing to their strong alkalinity [10]. In particular, previous studies did not use large portions of BFS in concrete.  
In this study, an alkaline solution was generated through electrolysis and used as mixing water to improve the low initial strength of BFS and address challenges related to alkali activators. The initial strength of the mortar was experimentally verified by improving the initial hydration reactivity of BFS using alkaline water, which was electrolyzed using potassium carbonate (K2 CO3) as an electrolyte. The experimental results show an increase in carbonation resistance.