###### Abstract  
Three-dimensional printing is an emerging technique that has received significant attention in the construction industry. This study presents an investigation into the printing and hardened properties of 3D-printed concrete (3 DPC). Both fly ash (FA) and ground granulated blast furnace slag (GGBFS) were used to replace cement in different ratios (0%, 25%, and 50%) to produce 3 DPC. Extrudability and buildability tests were performed to evaluate the effects of FA and GGBFS in various proportions on the printing properties of 3 DPC. Additionally, the hardened properties of 3 DPC were determined. Test results show that all mix designs meet the printing requirements of 3 DPC. The specimens with a higher proportion of GGBFS exhibited higher unit weight and compressive and flexural strength, but lower water absorption and drying shrinkage. The compressive and flexural strength of 3 DPC in the printing direction were the highest, outperforming those of the cast specimens at the age of 28 days. Our results indicate that FA and GGBFS can be used to replace 50% of the cement in 3 DPC. 3D-printed concrete (3 DPC); printing properties; fly ash (FA); ground granulated blast furnace slag (GGBFS) 2  
rheology and mechanical performance [13; 14; 15]. Thus, it is essential to develop appropriate 3D-printing materials. In contrast to traditional concrete, 3 DPC produced without formwork is designed to have low- or no-slump characteristics [16; 17; 18]. This results in adequate stiffness to preserve the stability of the printed shapes. Given the distinctions between traditional construction methods and this new concrete printing technique, it is crucial to take into account the fresh properties of 3 DPC in relation to the innovative nature of the printing process. Ordinary Portland cement (PC) is typically the principal cementitious material used in 3 DPC, constituting approximately 15-45% of the total mix [19]. The content of cement in 3 DPC is higher than that in traditional concrete [20]. However, the increase in the amount of PC may lead to higher material costs and severe environmental impact [21]. As sustainability concerns grow, researchers have explored the partial replacement of cement with supplementary cementitious materials (SCMs), such as fly ash (FA) and ground granulated blast furnace slag (GGBFS). These industrial by-products not only improve the sustainability of concrete but also enhance its mechanical and durability properties [22; 23]. FA is known to improve workability and reduce the heat of hydration, which enhances fresh concrete's pumpability and buildability. Similarly, GGBFS has demonstrated the ability to improve buildability due to its finer particle size and latent hydraulic properties, which can also lead to reduced cracking during the extrusion process [24]. However, both materials affect hardened properties differently: FA generally contributes to long-term strength gain due to pozzolanic reactions, while GGBFS often enhances early-age strength and overall durability [25; 26; 27]. Previous studies have demonstrated that incorporating these materials can positively affect both fresh and hardened concrete properties, but their impact on 3 DPC remains an area of active research. Given the unique requirements of 3 DPC, including the balance among pumpability, shape stability, and strength, understanding the effects of FA and GGBFS replacement on these properties is crucial. While some studies have explored SCMs in 3 DPC, there remains a need for comprehensive research on how different replacement ratios influence both fresh and hardened properties. According to research trends, there has been a focus on incorporating industrial by-products to reduce reliance on cement while minimizing waste and environmental impact [28]. [PERSON] et al. [29] found that the inclusion of nano-fly ash (N-FA) significantly increased the strength of the mix design for 3D printing. [PERSON] et al.